You could add a second default route to your second ISP with a metric
larger than 0. This would make this route less preferred than the first
default route.
I did a quick scan of /etc/sysconfig/network*, but did not see any
provisions for a second default gateway. A simple solution might be to
add your second default route in /etc/rc.local (man route).
That solution provides failover if the main default route goes down, but won't provide load balancing. In order to provide load balancing, you must use advanced routing and tools like iproute2 combined with MARKs.
This could help: http://www.linuxguruz.com/iptables/howto/2.4routing-15.html http://davidcoulson.net/writing/pcp/180/masterclass-linuxexpert.pdf
