How do I give 2 parameters to programs in an unix enviroment?

[Lasse Edlund]

If I have two files "foo" and "bar" and try to run diff on them I write:
$diff foo bar
I can also write
$cat foo | diff - bar
But how do I give a program two (2) commands? not only to diff
but to any program that wants double input...
I wanna do
$cat foo | cat bar | diff - -
especially with echo commands that would be handy so I dont have to
create files!