I haven’t done this, but you should be able to create VLAN(s) to a managed/smart switch which would go to the wi-fi access points then use the Firewall to forward/masquerade the VLAN to the respective gateway.
iptables -t mangle -A FORWARD -i eth0 -o eth1.11 -j MARK –set-mark 4
If needed you can put the 2 gateways on their own VLANs and create chains to pass the packets to and from the two VLANs
This is probably the simplest way to go about it. You will have to write the firewall chain to fit. It obviously depends on your network configuration and needs.