yes you can... they make little splitters to do such..
also you can get a stereo female jack and two mono
male or female jacks from radio shack.. some of the
spliiters can also be found at radio shack... best place
is an electronic - electric supply store..  not sure what
is in your area... we have "Randolph & William's" here
locally..... just go to yellowbook.com and search electrical
supply and electronics supply in your city.. 
if you try to make one up, it's simple.. just remember
on the stereo jack, the tip is left, the first shield is right
and the back of the shaft is ground..  tip is always the
positive on mono and left on stereo...

blessings,

bluey...

p.s. anyone please correct me if I'm wrong on the left
and right of the stereo plugs..

They are traditionally called Y-Cables.

http://www.musiciansfriend.com/navigation?q=y+cable&st=

yes this can be done very easily. all kinds of rewire splitters and cables out there for just about every application  you could thing of .. or you can build your own,... if you know how to solder wires. to cable ends