their are two types of interlocking via contactors. One is Electrical and other one is Mechanical.
Mechanical interlocking is very good and their are no chance for short circuiting. However electrical interlocking is also very much good and required fool proof circuit.
For electrical interlocking you need one spare Normally closed contact on both contactors, Contactor-1 A1 supply must be connected to NC contact of Contactor-2 and than to supply, same will be for Contactor-2 via NC contact of Contactor-1.