Stacked or Not?

It doesn't matter, just make sure you keep one on channel 1, and the other on channel 11, and you shouldn't get too much interference.

One does wonder why you'd need two wireless routers right next to each other, though.  

Thanks for your reply

I happened to get 2 for free..I sent one to Linksys as a warranty exchange, and they messed up somewhere and sent 2 back  ..So i just have 2 now and I needed the extra anyways lol

For the best setup I would recommend sitting the next to each other for heat issues and to reduce the amount of induced noise. I use to sit my wrt54g ontop of my linksys modem and once I moved it to the side my bandwidth increase due to less errors caused by electrical inductions.