latching circuit how to do it

To do it very simply, purchase a SPST switch that toggles push ON, push OFF. Someone like MCM Electronics has them. Wire that switch to the coil of your switching relay. The switching relay has as many poles as the number of wires that you need to swap. When ON it powers the power relay, the other position drops power to the power relay. The power relay has to be at least two pole, pole one (normally closed) powers circuit 1, the other pole powers circuit 2 (normally open). If you're swapping audio signals, you will need more poles. Wire your power relay so that the main circuit is powered when there is no power to the coil of power relay. This way you don't have to maintain power to the power switching relay to use the main circuit.