The PIX can't do any load sharing or similar to two different ISP's. You have to be very careful that traffic that goes out a particular interface in the PIX comes back in that same interface, otherwise it'll be dropped. You can only have one default route in the PIX also.
You're best off connecting the two ISP circuits to your outside router and then just create a small subnet in between this router's inside interface and the PIX's outside. Let the router do all the load-balancing between ISP's, etc, and the PIX will just point everything to the router.
