Depends what you are trying to do. Look up Eithernet Bonding for server-related stuff, but if you are just trying to get faster net for your PC, doubt it will help.
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Depends on your technical skill & budget. It's possible, but using for instance two 10Mbps connections won't double it to a 20Mbps speed unless you can use ADSL bonding (1 virtual connection composed of 2 or more physical connections). You'd need a fancy Cisco router or something similar though, those aren't cheap. If you use load balancing it will be as fast as the fastest line.
I'm in the US.
In my biz, we have some remote equipment we communicate to with cellular modems over the internet. We have a vendor that is using a cheap programmable router and 2 digi cell modems - 1 AT&T and 1 Verizon. He is load balancing and maximizing connectivity. Not sure how to maximize speed, but there is probably a way.
I do not recall the brand of router being used, but it is in the neighborhood of a few hundred dollars - way cheaper than Cisco.
You can't "combine" them as such, but you can load balance and take advantage of fail over with a load balancing switch.
They are expensive and you may not even notice the load balancing happening unless your were caining very large torrents on a very high speed connection. You're not going to notice a difference browsing or watching YouTube.
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