Yes, you can have multiple outside ISP interfaces but the problem is that there is no way how the PIX would detect that the first ISP link is down if the link is not physicly down. For example if the ISP has troubles a few hops away.
But you could alway change manually the default route , or shut down the fist interface so that the second route would be used or use the BGP routing protocol with your ISP.
Last way is to purchase a Link Controller (Loadbalancer) that is able to do that directly insted of doing this on the Firewalls.
http://www.elfiq.com/
http://www.elfiq.com/Webcast/en/LinkLBOverview/player.html
sincerely
Patrick