I dont have any experience with this, but I'd assume a 1:1 and setting the secondary WAN to be active would be able to achieve this.

 

Ironically this is what load balancing does... it would detect excessive load on the primary wan and port some traffic to secondary wan.  

 

One problem you'll have is that you'll show up as two totally different IP addresses, even to the exact same server, so theres an option to use an IP combiner service that would make your IP the same again.  This is also for fault tolerance, when primary WAN drops, the TCP packets can be rerouted to the second WAN can pick up all traffic and the source IP address remains the same.  I'm guessing this service would cost $10 or so/month.

 

Would be interesting to see info from somebody actively doing this.

 

Of note, after Hurricane Ian rolled through Central Florida, we had internet via Spectrum cable, out for several days.  I went to TMobile for their internet offering and plugged it into the secondary WAN port and did some configuration to get the network up and running.  I was expecting Spectrum to be down for several more days but Ironically the Spectrum cable connection came back online a few hours after the TMobile device and I didnt even realize the gateway had shifted all traffic back to Spectrum.

If only this was possible! I would like that, but I don't see it being feasible...