Hi!
The first thing to do is to never have the engines stop and second to build light airplanes! Light airplanes fly better and will not stall as easy.
So setting the engines correctly is very important. so practise, practise. You must be able to set your engines by ear! Pylonracing is a good learning ground.

If one engine stops despite you being good at setting engines....chop the trottle instantly and begin to decent (keep the airspeed up) and land. Never turn if you're flying low...and be prepered to dive to maintain airspeed.
If one engine dies when you are taking off, Ã*n a nose-up attitud, chop the throttle and fly straight and land. Don't try to turn sharply or give up elevator! This will mean instant snap roll and crash.
Keeping the airspeed up is very important!