Well, you can just start an exact solver on the problem in one thread, and an approximate algorithm on another. If the former doesn’t finish in n seconds, you cancel it and use the result of the latter.
The advantage of combining them though is that you might be able to treat different subparts of the problem differently (which is the hard part), so that you could use an approximate algorithm for the hard part of it, and an exact algorithm for the easy part.
Thi of course assumes the problem can be divided in this way, which is fairly speculative.
Thi of course assumes the problem can be divided in this way, which is fairly speculative.