How Do I Configure an Inside (private LAN) Router to Route Through Parallel Firewalls to the Internet?

[admin_2]

Having two default gateways (0.0.0.0) causes the inside router to load balance.

How does the perimeter router (outside) decide which router to send replies

back through? With stateful inspection running, it has to go through the originating

firewall. How do I do this?

[#2]

You need to use a Cisco CSS11000 load-balancing switch in front of the firewall.

See the example at:

Configuring Firewall

Load Balancing on the CSS 11000

Also check: 

Configuring

Firewall Load Balancing