Not sure about the Firewall part you did, but to forward packets between two networks through a linux box, you have to enable ip_forward in it

# echo 1 > /proc/sys/net/ipv4/ip_forward

This enables the ip forwarding till next reboot, to make this packet forwarding permanent

# vi /etc/sysctl.conf
change the line net.ipv4.ip_forward = 0 to net.ipv4.ip_forward=1.

This should do the stuff, to check if your iptables rules works correctly.
First turn the iptables off
# service iptables off
#iptables -F

then try pinging between the boxes, if ping is successful, then turn the iptables on and edit the rule as needed, and verify.

Meanwhile, i will try to figure out the rule which will help you.

Cheers
DD