Must have some seriously terrible speeds on the main connection to
make this worth while; i.e sub 100mbit.
The hassles with port forwarding/per connection tracking and having to
then optimise latency sensitive traffic using rules to prefer wired vs
wifi uplink etc really make this pretty niche use-case.
The only valid reason for this would be to have a backup path; but
then again it's using the same providers network so it's not a
different AS anyway.
If you are using Multi-wan for additional speed ; then you should
probably think about getting a new provider/plan.