The statistical likelihood of being bumped twice on the same itinerary is probably fairly low - though I'm sure SOMEONE will now post a response saying that it happened to them!!!

If you can, I would contact LTU beforehand and ask them to put a note in your booking record to say that you have a 3.5 hour long-haul connection at FRA - this won't guarantee that they won't bump you, but it'll make it less likely that they will.

The other thing is that you should perhaps try to get to the airport MORE than two hours before departure (hard, I know - especially at that time of day!), so that you are not at the back of the queue. My experience with overbooked flights is that those at the front of the queue will get asked if they would like to volunteer to travel later, but if they all say "no" then those at the back of the queue will be TOLD to travel later. So make sure you're at the front!