Question about signing up to someone else....

Let's say I sign up to Three Broadband during the 14 day cooling off period for Virgin Media, am I able to just have both Broadband running at the same time if I a) plug their router into another mains socket or b) unplug your hub 5 from the mains, plug in theirs, and then plug yours back in? Can I effectively just switch back and forth like this or once I plug in their router will it can your connection? 

They said their router doesn't need a wall connection like VM does, and that their outdoor hub just goes outside the window, and their inside hub just plugs into the mains? I specifically asked them if signing up would adversely affect the line with VM and they said no it wouldn't. This is extremely important obviously 

Just checking from your side of things. I would be in the cooling off period for both for about a week