Yes as Rolli said you would have to get a motherboard with two CPU sockets but there are a few things to remeber:
1) Dual CPU's motherboard's will be pretty expensive and so will two CPU's.
2) If you are thinking about this for gaming don't bother. Let's say you got two Intel quad core Xeon's, yes you have 8 real cores but it's pointless as games will not make use of the 8 real cores.
3) Most dual socket motherboards are going to be made for servers so they are going to use buffered ECC RAM which is a little slower than unbuffered non ECC RAM.