You can only have one visible server form on the page at a time.
So you could have more than one server form as long as you only show one at
a time.
Or you can have more than one form visible on your page at a time, but only
one of them can be a server form (i.e. with the runat=server attribute.)
Server forms only support posting back to themselves (not to other pages.)
It's my understanding that this will become more flexible in .NET version 2.