For effective purposes, the term is load balancing.
For Internet usage, both your router and ISP's hardware must support it.
If you just have two dsl lines in house, connect to both (you are limited to wired connection for both, since windows hates wifi+wired at same time)
then alter routing tables so that windows will prefer certain IP's through one or other connection.
Yes, you will need to do that IP by IP basis, or IP ranges which will not really work for internet usage.
It could work with watching netflix on one and browsing other websites with other since... netflix has only limited amount of IP's in use.
Past that, only real benefit with having two connections (besides total hassle managing said IP routing tables) would be that computer prefers one or other normally and IF one drops down, it will pretty nicely just use the other.
it will NOT double your download(upload speeds though since anything will go through network A or B
to double speeds, you need load balancing
http://www.ezlan.net/loadbalance.html
https://speedify.com/blog/combining-internet-connections/combine-two-dsl-lines/
Edit:
Following would allow you to use both at once, kind of. It works on stream by stream basis so based on said streams bandwidth, it either uses A or B for it. It will never give you A+B though. (so two 20Mbit connections will still give top speed of 20Mbit but you can have two 20Mbit connections to different sites)
https://www.linksys.com/us/p/P-LRT224/