Here is the net-all solution to the problem:

Do the dungeon regardless.

If you have a tank who can't tank 2 mobs, just run as if you have no tank.

If you have a healer who can't heal, just run as if you have no healer.

It can almost always be done. Now if you have neither a tank nor a healer who, and only two DPS, things get significantly harder. Beyond what a random DF queue can likely pull off. And frankly it would virtually require two ranged DPS.