Bonding and redundancy

I got two backhaul towers connected to each other via a 80Ghz radio link.
The radio has two interfaces (eth2 and 3 on the MT) which I bind using bonding 802.3ad with mii and transmit hash policy layer 2.
The bonding interface (bonding1) I put in VLAN10.
VLAN10 I added to bridge1 with my public IP on bridge1.
During heavy rain, the 80GHhz rains out and takes about 5min to reconnect.

What I want to do, is to add another lower freq radio to the same bridge so that when the 80Ghz drops, the 2nd radio will carry the traffic.
How can I do that?
Thanks