Some info before you proceed down this path
1. Failover takes a little bit of time to activate. The device with both wan connections has to know that primary wan is down. If it's a media disconnect (interface unplugged, turned off, loss of signal), the detection is pretty much instantaneous. However, if it's a high latency/soft fail (immediate upstream router is up but connection is failing beyond that), the multi wan router will take a while (seconds) to detect the failure and switch over to the alternate wan. This is good enough for everything apart from realtime things (calls, games, live streams).
2. Your IP changes when failover is triggered. Most websites don't care. Banking (and similar secure) websites sometimes do. Sockets are disconnected and have to be reconnected, might not work in some cases (multiplayer games).
3. "double the bandwidth" also isn't that simple. A single client connection can only be routed over a single wan, but multiple connections can be routed over different wans (so file downloads over a single connection will be limited to a single wan's speed, but multi connection downloads like idm or torrents may benefit). For load balancing, if your client IP to the same server changes every other request, they may take offense and reject connections (similar to banks). Some routers like microtik have a feature where they can use the same wan for all the connections to a specific destination (PCC?).
Knowing all this, the actual setup of multi wan is pretty straightforward. I use pfSense, but tplink and microtik has multi wan routers that are relatively inexpensive. You need 2 wans, designate them as wans and set a failover with a priority order.