Running Two REL Subs off One Amp - Possible?

I'm thinking about using two REL subs to see how big an improvement they make over a single sub for two-channel music. I'm concerned that I can't wire both subs to my single amplifier, though. Anyone know if this can be done?

Side issue: I had a dealer (not REL dealer, though) tell me the best way to use stereo subs - in most cases - is to put them in opposite corners (ie front right and back left) of the room to even out the bass throughout the room. He also stated the only way to increase output over a single sub is to put the stereo subs on top of each other or side-by-side. Is this good/correct advice?

Thanks,
Howard