From experience I would never recommend it. Wedid exactly what you are thinking with our previous tt and 2 adult bikes. We bought one of those bolt on hitches and away we went. There was a lot of play in the receiver and the bikes bounced around a lot. In short time I noticed a couple of cracks in the welds between the bumper and frame. I had a welder beef things up with some brackets and gussets. Before long they too were cracking and the steel bumper was tearing.
The only good solution is to have a hitch fabricated that welds directly to the frame rails. It only cost us $250 and was solid. We did have a hole drilled through the bottom of the receiver and a nut welded over it so we could thread a bolt through to tighten against the bike rack and take the slack out of the hitch.