What the pix cannot do is take traffic arriving on one interface and send it back out the same interface, and it does not matter what interface it is (inside, outside, perimiter). In your topology the traffic from network a will arrive on one interface and will be sent to network b via another interface, so this should work.
What you need to do is to modify the existing vpn config on both pix units to allow the 2 networks to see each other, and you may need to bypass nat but that will depend upon your security policy, dns/wins info among other things.
Let me know if this helps. In particular check that I understand your topology correctly - the pix interface at the corp. office that the vpn connection terminates is the outside interface (same one used for inet access), and that traffic from the corp. net and remote site a enters the pix on the inside interface.
