routing between vpn sites question

hi,

I have a problem and dont know how yo configure my cisco pix.

the cenario goes like this:

I have two remote sites, lets say A and B, they both have cisco pix
and are connected to the corporate pix throw vpn over the internet.
from site to site B I want to be able to connet from site A pc's to
site B pc's and visa versa. today site A pc's can connect to the
central conrporate machines and site b can connect to the corporate
computers,
I think I need to add a route on the corporate pix which is 515E but
the two vpns are connected to the same external pix nic so what to do?
and how?

Thanks a lot !!

Juan