What if Batman gave Thor the kryptonite and Thor ripped Superman apart with it? Captain America could do the same by giving Superman his adamantium shield, but Captain America only has ONE weapon whereas Batman has tons. Assume that both street level fighters are tossing their weapon to their receiving superpowers; Batman would deflect the adamantium with a crapload of projectiles and hand his kryptonite to Thor, who would then rip Superman's skull apart.
Now your makinging me argue for the other side. How about Batman goes to hand Thor the K, but realizes his hand has been melted off by heat vision? Thor wants to kill Cap, Cap might be able to run around for a lil' while and block w/the shield before he's dead. Supes wants to kill Bats, Batman's neck is snapped like dental floss in-between heartbeats or he's melted from a distance. Supes can run to the shield and f**k Thor up with it faster than Thor can get the K and f**k Supes up with it. & Batman would still be on the ground dead before Thor can say ouch. If this is "Which side can make it a 2 on 1 quicker by executing the weakest guy on the other team", that goes to Supes & Cap easily no matter how much Batman knows about Supes. Understand? I still say Thor fries everything 'round him with lightning & it interupts Supes Speed blitz and takes Supes out. Thor is durable enough withstand a few Supes punches and still use magic on SupesWrong. The PIS that happens is PIS.