You don't understand how it's possible to win with Back-Back bets
At first this might be confusing, since you might be thinking than covering a back bet with another back bet, just nullifies both. And yet you have to invest money in two separate bookmakers?!?!
Here are some concepts that will help you clarify this:
You are able to win, because you use a bonus. Usually when betting your own money with the system will result in a small lost, but this way you complete the bookmakers' requirements to get the bonus, and from doing the same again with it, will result in having a profit.
You need to understand well how back-back bets work. When betting this way, you have to make sure to place the bets on 2 outcome markets, doing the second bet on the opposite result of the first bet you made. This way, when one of the outcomes happens, you will have a winning bet and a losing bet. When you do this with the bonus on one of the bookmakers, since you're not investing your own money, you will make a profit.
This is a simplified explanation on how Matched Betting works, but to truly understand it, the best way is to complete the Welcome offer, that will allow you to win between 120$-140$.