I don't know why not.

Fred

Sure.  If both phones are locked to Cingular, you should be able to switch
SIMs between them with no problem.  Check it now and see.

In order for that to work in Europe, he will have to have international
roaming on his account.  That is expensive.  Depending on how much calling
he is going to be doing, he might want to use a local SIM.  That requires
that he unlock your phone...  Now that gets tricky, too.  Your phone would
need to be unlocked, which Cingular says will invalidate the warranty.

If you ask, they should send an unlock code via email for free, or you can
pay several local vendors about $20 to unlock it.

He could take his phone to Europe, where he could still use 1800 coverage.
I don't know how much of a loss that would be.  The suggestion about
unlocking now applies to his phone, so warranty and other potential
physical troubles are his, not yours.

http://www.cingular.com/customer_service/cingular_world
http://www.gsmworld.com/roaming
http://telestial.com/prepaid_sim_cards.htm or others.