how i can add defrent getway in defrent bridge

hello
i have router 1100Hx2 lv6.
In interface wan1 adsl modem 192.168.1.1
In interface wan2 adsl modem 192.168.2.1

i have two bridge
bridge_a ,eth2,eth3,eth4,eth5
bridge_b eth6,eth7,eth8,eth9,eth10

how i can make default getway for bridge_a=wan1
how i can make default getway for bridge_b=wan2

thank you for help me

Mark traffic in NAT.

Start by reading this. It isn’t exactly what you want, but it will help you understand the basics of how to do this:
http://wiki.mikrotik.com/wiki/Per-Traffic_Load_Balancing