With Both tunnels you mean you have two WireGuard tunnels to Mullvad?
If so do not Enable route allowed IP's but do use 0.0.0.0/0 for allowed IPs
If done so there is no default route and you can use pbr to make routing decisions and your main routing table is using your ISP.
You might have a DNS leak if that is a problem see: https://github.com/egc112/OpenWRT-egc-add-on/tree/main/stop-dns-leak