I have read all the answers given and I think you should be able to do it fairly easily on a single subnet. I did this when I had 2 lines both with ADSL. Basically Each line went to a linux box which also acted as a firewall, I also ran a dns server on one of the linux boxes which decided who connected to each line. Both linux boxes were on my internal network.

Snags

1. 2 linux boxes running 24hrs a day (mine were just old computers)
2. had to muck about in the DNS server if you wanted to change a user to the other line.

It was very useful for a period when we had 2 lodgers and limited data (40GB per month) it did also provide lots of redundancy, however now with FTTC we just have the one line, one box running 24hrs a day and all the other computers are shut down when not in use. The saving in Electricity is amazing. We were using aprox 8500 units per year now with solar panels we are down to about 3500. Happily very few glitches on the FTTC line. If that dies now I would try and use the hotspot on my mobile for essential connections.

Draytek allow you to have 2 ISPs and have a feature called Lan IP Alias
So you can manually set some devices on the same subnet with a different gateway.

Have a look here.

https://www.draytek.com/support/knowledge-base/4917