I realize that a limit of a single hand to hand is implied by omission since no classes or OCCs started with more than one (I guess the exception being the Dedicated Martial Artist in Ninjas and Superspies and the Open Hand Martial Artist in Mystic China) but I'm looking for explicit declarations, not implications.
In the Palladium RPG Revised, most OCCs only had one hand to hand available, but there were multi-classing rules on page 16. The example given, had me thinking a Thief could bring Hand to Hand Thief to 3rd level (page 22, 2 attacks +2 parry/dodge) an then when he became a 1st level wizard, buy Hand to Hand Non Man at Arms (page 21 +2 to dodge only) to get a total of +4 to dodge. So now on to later books which would later explicitly condemn this line of thought:
There are a couple which I think some people take to be be nails in the coffin, but which I don't, but I will print it for discussion purposes. It appears as a disclaimer in the physical skill sections of:
Rifts page 28
Nightbane page 52
Macross II page 22
Palladium Fantasy Second Edition page 55
ALL attribute and skill bonuses are accumulative. However, a specific physical skill may only be chosen once, including hand to hand skills.
I checked Robotech Book 1 Macross page 26 but didn't find any explanation in the physical skill section, just assigned cost of 1/2/3 for basic/expert/martial
A similar statement appears in Heroes Unlimited 2nd Edition page 55:
All attribute and skill bonuses are cumulative. However, a specific physical skill may only be chosen once, including hand to hand combat.
Basically nearly identical except for capitalizing "all", the "ac-" prefix, and "skills" v "combat" at the end.
My view is that each hand to hand skill is specific. These statements prevent buying hand to hand basic twice. I do not believe they prevent buying Basic and then buying Expert. Now on to later books which do appear to have a stronger argument against the practice.
Strangely, a much clearer statement is found January 1993 in the earlier Heroes Unlimited Revised (9th printing page 33) which I also saw in January 2000 (Teenage Mutant Ninjas Turtles & Other Strangeness Revised Edition page 22
Only one hand to hand skill can be selected and a skill may only be chosen once.
The first strong argument I've ever seen in any book against stacking multiple HTH skills. Not because that explicitly isn't allowed, but because you could never have more than one hand to hand skill, so whether or not you could stack them is a moot question. See also ATB below.
May 2000 Ninjas and Superspies Revised 7th printing page 82-3 (though this has probably appeared in the earlier editions too)
Here's a few important things to bear in mind when selecting Martial Art Forms.
1. Each form must be used separately.
This is clearly just a statement about FORMS though, so I don't think it has any hold over hand to hand skills in other games.
May 2001 (After the Bomb RPG page 49)
Each skill may only be chosen once, and only one "Hand to Hand" skill can be selected.
This along with the earlier HUR/TMNT statement are the 2 strongest I have ever seen ANYWHERE. I concede that there is absolutely no way at all to select more than one hand to hand skill in either the TMNT or After the Bomb RPGs. It's very clear. So it's basically the other non-Wujcik books which are up for grabs.
This doesn't actually prevent stacking multiple HtH bonuses, it just makes it a moot question since you could never have 2 to stack.
September 2001 (Rifts Game Master Guide page 32)
Can characters have more than one Hand to Hand skill? If so, are the bonuses cumulative, or are only the best bonuses chosen?
Yes, conditionally. Other physical skills like Boxing and Wrestling offer additional bonuses to attributes, S.D.C. and combat bonuses (strike, parry, dodge, etc.). ALL these bonuses are cumulative. However, all Rifts® characters only get ONE Hand to Hand Combat skill.
I will point out that the answer IS yes. The phrase "Other physical skills" clearly highlights that hand to hand skills are still considered physical skills.
The "only get ONE" statement's meaning depends on how you read "get". I see this as referring to assignment, as in hand to hand skills assigned by OCCs. Indeed, you only ever "get" 1, I've never seen an OCC which STARTS with 2 hand to hand skills by default.
I would view a distinction between what you "get" and what you "buy"/"select". Spending OCC related or secondary skills to obtain a second hand to hand, for example, doesn't fit the implications of "get" because it wasn't given automatically, you had to make a choice and sacrifice options to select it.
the only exceptions are characters in the Ninjas & Superspies RPG where some martial artists characters can select multiple forms of hand to hand combat training
This subsequent parenthesis may lead some to assume it to be condemnative, but I don't see it that way. Viewed in the above context, I think it is only talking about starting automatically with multiple hand to hand skills.
This is the case for the Dedicated Martial Artist / Open-Hand Martial Artist. They start with 2 forms. This is by default how it works. The only exception (and only in the case of Dedicated) is if you opt to take a single exclusive form (which may have originally just been Ninjutsu/Thai in older versions but I think is all Exclusive in later ones).
These are indeed the only 2 OCCs that I have seen which automatically start with 2 kinds of combat skills.
Being the only OCCs which start with something does not necessarily mean you are the only OCCs capable of learning it though. There's a list of OCCs which start with WP sword and then many more OCCs which can select WP sword by spending skill slots. I believe hand to hand is the same way.
October 2004 (Splicers page 95)
January 2005 (Beyond the Supernatural Second Edition page 206)
August 2005 (Rifts Ultimate Edition page 316)
March 2008 (Robotech Shadow Chronicles page 268)
November 2008 (Dead Reign page 205)
There are five choices for Hand to Hand Combat skills available to the character (pick one)
This isn't actually as strong as it could be. "Pick only one" or "Pick a maximum of one" would have been better if that were the intention. I can pick flowers 1 at a time to get 2 flowers, after all. But at least it's something. The statement in GMG is much stronger, so people who take an "Ultimate only, forget what the Game Master Guide says, it's outdated" kind of stance should reconsider that since the GMG is the best argument against HTH stacking I know of in RIFTS (even though, as above, I still think it is inadequate)
I've mainly just looked in the physical skill sections since I couldn't find any disclaimers adjacent to the lists of bonus charts. If there are other statements elsewhere in the books I'm hoping people could introduce them.
So as a final thought, as above, the Heroes Unlimited Revised Erick Wujcik's books of TMNT/ATB have the only explicit statements I could find preventing multiple Hand to Hand skills, negating the opportunity to stack them. His book of N&S also has the only statement about using one at a time.
I don't own earlier editions of TMNT/N&S so I'm not sure if these were added later in revised editions or if they were present from inception. If anyone owns earlier copies than January/May 2000 perhaps you could clear that up.
The other books lack 100% explicit statements preventing the simultaneous owning or usage of hand to hand skills. The IMPLICATION that you can only have 1 is certainly present by omission, as I stated previously.
It's very interesting though, that the very clear "Only one hand to hand skill can be selected" was REMOVED from Heroes Unlimited Revised. It doesn't appear to be present in Heroes Unlimited 2nd Edition. Implying you can buy multiple hand to hand skills, just not 2 of the same one.