So I read through the new GK codex and surfed around the net a bit and couldn't find the answer to a couple items. Even if the answer isn't clear, maybe you guys could give me some insight into the ways it has been handled in the past.
1. The Hammerhand psychic power - Can the +1 S from this power be stacked upon one unit? It explicitly says that Might of Titan stacks with Hammerhand, but can you put Hammerhand on one unit 3 times?

2. On a similar note, can Rad Grenades stack? If you do a dual assualt with two units that have rad grenades does the -1 T from each unit stack? Or if you have two IC's with Rad Grenades attached to one unit that assualts, would it lower the T by 1 or 2?

I tend to take the more conservative stance on these items, but it has a pretty large affect of list building and game play, so I thought I would throw it out there.

My initial impressions are yes for hammerhand. No for rad grenades. This will be in a FAQ in a few months inat or gw. Regardless, after the FAQ I bet that hammerhand will not stack at that point.

Supporting case: If I cast fortune twice on seer council does the reroll armor saves stack? No

I'd argue that since Might of Titan explicitly says that it does stack and Hammerhand does not it won't stack. I know that's some sort of argument by absence, but it still makes sense.

Store ruling is good and all but I'd play it not. Every game in a tournament would be a Rules dispute for you otherwise. An that equals lost sportsmanship. Not worth it.

Honestly you can really only double up with independent characters so prolly a max of 2 units. Not really game breaking anyway.

5, Techmarines. But your point remains valid, and honestly you don't really need it to stack.

So I spent some time looking at the codex specifically with this in mind. The idea of putting 2 ICs in a unit for 2 hammerhands is just silly and costs too much. You can do almost the same combo with one IC. Rad Grenades and Hammerhand gives you the same results and the 2 point swing in toughness is all you will need. And you won't have to worry about Faqs.

I have a list coming up on the blog in the morning centering around screwing with the enemies toughness and your strength in 4 of your hand to hand units.

Edit; I'm talking henchmen btw

The only way I can see stacking multiple Hammerhands is if you have a Librarian or Inquisitor join a unit that already has Hammerhand.

Unit activates Hammerhand, Librarian activates Hammerhand and Might of Titan. So if you have an Inquistitor as well, Hammerhand for the 3rd time? Why not? So with this logic if I can get Might of Titan off twice in the same unit do I now also get 2d6 penetration instead of the one d6? It doesn't say that Might of Titan stacks with itself just as it doesn't say Hammerhand stacks multiple times.

Obviously there is no question about Might of Titan and Hammerhand stacking, but stacking Hammerhand multiple times seems a little op to me. But if there is no rule to say it can't happen, I guess it is legal until a FAQ says otherwise? Is this how it should be played until FAQ?

I would assume no stacking of Hammerhand, based on precedent from the Space Wolf FAQ on Frostblades:
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1620228a_Space_Wolves_Version_1_1.pdf page 4:
Q. Does a model armed with two frost blades/axes get +2
to their Strength? (p57)
A. No.

I don't have a copy of the Grey Knights codex (and didn't think to look at this on Saturday), but what does the wording on Rad grenades read? If it's "A unit assaulted by a model carrying rad grenades is -1 toughness until end of turn" or something to that effect, they wouldn't stack, because they would all check the same qualifier. If it said "models in base with this model are -1 T" then you'd have an argument for stacking.

Wording for Rad Grenades:

During a turn in which a unit equipped with rad grenades launches an assault, or is assaulted, the enemy unit(s) suffer a -1 penalty to Toughness until the end of the phase(this does effect the victims' instant death threshold).

I'm reposting this from the other thread, that way it can be kept with the relevant discussion:

sirhelnor wrote:

I could be way off base here, not having been able to read the rulebook closely, but do rad grenades stack if multiple units with grenades assault a single unit? or is there a unit preventing this? if not, what is preventing a unit of T3/T4 models from being assaulted by 3/4 units with rad grenades and being instagibbed at T0?

I don't see anything that explicitly says that they stack or don't stack. It should be addressed in the FAQs, but until then it will be a point of contintion. I think it is rules lawering to stack them, and the more reasonable approach is to not go that route. Typically, GW makes it clear if they intend on something stacking, like they did with Hammerhand.