Lets say R1 is connected to ISP1 & R2 to ISP2.
you create a route-map with 2 sequences on R1.
sequence 1: For subnets coming from ISP1 subnet, fwd pkts to directly connctd serial interface of ISP1.
sequence 2: For subnets coming from ISP2 subnet, fwd pkts to vlan1 of router2.
Similarly, you create a route-map with 2 sequences on R2.
sequence 1: For subnets coming from ISP2 subnet, fwd pkts to directly connctd serial interface of ISP2.
sequence 2: For subnets coming from ISP1 subnet, fwd pkts to vlan1 of router1.
You are now ready with configs. apply the respective route-maps to respective vlan interfaces.