2 Leased line for 2 segment of LAN

i have two Internet leased line for my LAN say INTERNET LINE A & INTERNET LINE B. all the clients & servers are behind the PIX 506 firewall. now i want one segment of my LAN to go through INTERNET LINE A & another segment through INTERNET LINE B. both segment should go through FIREWALL.

both segment of LAN should be able to talk to each other.

What is the best soultion ? how do i perform this ?