VLANs are one way to do it. It’ll allow you to separate your clients traffic at layer 2, and then handle all layer 3 routing independent of each other.
If you have a large bridged network you’ll just have to build the VLANs out in your switches from your core to your access layer.
If you’re using a routed environment you’re better off using Virtual APs & adding them to a bridge along with VPLS, EoIP, or GRE tunnels from your wireless APs back to routers on your respective networks. Let each ISP handle their routing separately. You could handle routing directly on the APs, but it may start getting complex with two providers trying to manage layer 3 routing in a single router. MPLS is a solution to this problem, but it has a bit of a learning curve and various network requirements prior to implementation which may be too much to worry about for this relatively simple issue.