I've seen it happen a few times. Sometimes it was obvious what they were doing, other times it may have not been planned that way, but just ended up that way.
A JW guy that I grew up with and went to school with got married young to a JW girl the same age as he was. I think they were 19 when thay got married. As time went by it was painfully obvious that their marriage was not a good one. They were not a good match. After almost 20 years of unhappiness in their marriage, the brother, who had become an elder by then, started seeing a single sister on the side. At first it was "encouragement" since she was all alone in the "truth", but eventually they encouraged each other in other ways (wink wink, nod nod). It was eventually found out and his wife immediately left him and started divorce proceedings. He shacked up with the younger, single sister, they both got DFd. They eventually married and a year or so later, they both got reinstated.
I sincerely believe that he didn't plan the entire thing to the point of being reinstated and accepted back in the congregation to take up pretty much where he left off (he's an MS again, now). I think because so many JW kids married so young, that after a few years they realize that maybe they could have chosen better, but under JW law, they're stuck. I think some become desperate enough to cheat but without longterm goals of coming back with a new wife on their arm. Again, they're thinking short term just as they did when they got married.