GR's Mewtwo & Dark Clefable - stacking?

So, I decided to try out building a deck around GR's Mewtwo and Dark Clefable, after using the appropriate all-cards codes to get enough copies of everything. I'm noticing, though, that even though the descriptions on the Pokemon Powers for GR's Mewtwo and Dark Clefable make it sound like they should stack (meaning, if you have more than one copy of them, all should apply), they don't seem to do so. Can anyone confirm either way, whether these powers stack or not?

Note, this is not asking whether both GR's Mewtwo and Dark Clefable's powers will apply if I have both out. That does work. It's just that if I have, for example, two GR's Mewtwo, it always seems as if only one is applying its power.

GR's Mewtwo and Dark Clefable Pokémon Powers do NOT stack.

The most effective way to use GR's Mewtwo is to attack with it obviously, you shoud have 5 Dark on the Bench on turn 2 without any problem (Dark Slowbro helps you abuse Computer Search + Professor Oak). The only Dark attacker worth using in this deck is Dark Fearow in my opinion. 50~60 for a DCE is great, and it serves as a good backup if your GR's Mewtwo gets Energy Removal'd.

I'll have to try out the version you're talking about. Personally, I've been finding the Dark Raichu + GR's Mewtwo + Dark Clefable setup (Mr. Mime LV28 optional) to be pretty effective, at least in-game. True, Dark Raichu tops out at 50 damage to the active on Turn 2 (thank you, Pikachu LV13!), as opposed to GR's Mewtwo being able to average 70 (with a full bench of Darks), but the fact that GR's Mewtwo's Power does affect Bench damage makes up for it (though it is always a bit annoying when GR's Mewtwo chooses to do +20, only for Surprise Thunder to end up doing no bench damage).

Incidentally, has anyone figured out how the odds actually work out on the "random choice among three values" thing that Dark Clefable, GR's Mewtwo, and Dark Raichu all do?

Dark Raichu is as follow :
50% 0 damage on Bench
25% 10 damage on Bench
25% 20 damage on Bench

I will disassemble the game for GR's Mewtwo and Dark Clefable, if you can wait a day or two =)

I just began, I will try to figure out the probabilities of 0, 10, 20 tomorrow.

http://datacrystal.romhacking.net/wiki/Pok%C3%A9mon_Trading_Card_Game_2:ROM_map

Done.
Those are the exact probabilities for GR's Mewtwo Dark Wave and Dark Clefable Darkness Veil :

00 : 86/256 33.6%
10 : 85/256 33.2%
20 : 85/256 33.2%

http://datacrystal.romhacking.net/wiki/Pok%C3%A9mon_Trading_Card_Game_2:ROM_map#Pseudo_Random_Number_between_0_and_2

Interesting. So GR's Mewtwo and Dark Clefable approximate even chances, whereas Dark Raichu actually uses the same probability spread that would eventually be used when the card was printed in the real game. (The real Dark Raichu has you flip once to see if you do Bench Damage, then, if you do, has you flip again to determine whether it's 10 or 20.) I was actually half-expecting GR's Mewtwo and Dark Clefable to use the same setup as Dark Raichu, but the near-even spread does make more sense.