In PIX IOS version 5.2 and higher, you can have an interface other than e1 as the outside interface. Also, though not a Cisco recommended (or TAC supported configuration), two interfaces can have the same security level. Based on these facts, I think it just might be possible (though I have not verified it or come across such a setup). On the other hand though, you could certainly have an additional interface (say e2), name it something like 'outside-2', assign it a security level, say 5, and use it to connect to your second ISP. You can certainly load balance on these links using static routes. You could configure outbound traffic to certain IP's over one interface, while directing another set of packets, based on their destination IP, to the other ISP.
Strictly speaking though, this is not the best way of doing things. A better way of doing things would be to deploy a perimeter router (on the PIX outside interface) and multihome it. Then you could make use of a large range of Cisco solutions for load balancing over the links.
