Looking for help to choose router for linking two bandwidth connections together

There are a number of router than can do dual wan. If you really want to you can load third party firmware link dd-wrt on many different router and get the feature.

Still it will only partially do what you want.

It will work very well as a primary/backup still it requires some configuration. The large problem is what does "down" mean and how do you detect it. If you define a method you as a person would use to detect this you can likely get it done. The ethernet port on the device seldom goes down and even the first connection to the ISP may stay up. Most time people find something to ping on the internet but there are cases when you can get to some servers but not others. It tends to be tricky to get something that works all the time. This is solved on commercial connections with a routing protocol like BGP but no ISP will do that on residential connections.

Load sharing is possible to a point. The simplest is to force certain machine in your house out one and other out the other of course if you get a failure of either connection everything switches to the remaining connection. You can also send certain destination address or protocols. You could for example run youtube on one and everything else on the other. It does tend to get tricky if you just leave these routers to try to load share by themselves. For example a online game normally has a number of servers. One for login and account management and a bunch used to play the game. If you were to use one connection to the login server and then use another to access the server the game company would see 2 different IP and detect this as hacking. There is no software that can solve that issue you must manually tell the router which connections to use.

The part that is almost impossible is to actually combine them and get a larger single session. For example if you had 2 2meg connections and a HD youtube video needed to use 3m you still could never use more than 2m. You could of course download 2 2m videos at ones one on each connection.

There are VPN services that claim to be able to combine these but they do their very best to hide the fact that you now just get a different problem. Sure now all traffic comes from 1 IP but you have different delays in the actual connections to your house so now you get out of order data packets. Out of order data causes the end stations to retransmit data because they think data is lost, if it gets bad enough the session is reset and you must start again. This out of order data can cause massive issue for anything that requires smooth data delivery like games, video and voice calls. For filetransfers it works ok because they tolerate out of order data better.