How do you guys handle active-active internet connections with different datacenters and firewalls?

Just curious how you guys handle active-active internet connections with different datacenters and firewalls? I have two datacenters in different locations with a link to the internet (same ISP). Currently we poison one link and it goes unused but we're interested in an active-active design. However this causes issues with the firewalls being stateful. If traffic comes in side A, and then hits the datacenter on side B, but the upstream router on side B prefers the default route on it's internet link, then the firewall on side B will block the return traffic. Just curious how you handle that. Thanks,