I apologize in advance if this has been addressed before, but I couldn't find a thread on point.

Last night, my opponent tried the following: He had priority, and flipped Danger Room, and stated that he was putting its "front row X-Men characters +1" effect on the chain. Then he stated that he was flipping ANOTHER Danger Room, and putting the same ability on the chain. He acknowledge that he would have to KO the first Danger Room because of the uniqueness rule, but he argued that its effect was already on the chain, and that the effects of the two Danger Rooms were cumulative, such that when the two effects resolved, his front row X-Men would get +2.

Is this valid? In theory, it seems to conform with the chain resolution, but it also seemed to fly in the face of the uniqueness rule. Comments?

Danger Room does not use the stack. It is a continuous power. It starts as soon as it is flipped up and is gone as soon as it... gone.

The situation as you explained it would not work.

Now, if the location was one with an activated ability like savage land, then it would work like your friend wanted. Savage land has a payment power that does use the chain.