It's better to have 1 64-bit processor because it has the 64-bit technology in it. Also 32-bit processors are much less powerful even if you have 2 of them. 2 of them requires a special dual socket motherboard and they are not available anymore.
So the bottomline is: get 1 64-bit processor because modern day CPU's have 2 or more cores in it. This means you'll have several processors inside of 1 processor and therefore have more speed then 2 x 32-bit CPU's and it takes less space.