Hi, the solution is already in the normal window switches in a car, they use SPDT ROCKER switches.
Use one SPDT switch to do the job of both sw1 and sw2, then both switched outputs will never be on together.
Please check the normal power switches that you talk about and see what they do, if the basic system is already fitted, then it must have some way of interlocking all ready as part of the system.
Also use diodes on the arduino output, so that you can leave the arduino outputs as outputs and the diode stops the switch from feeding current into the arduino.
Sorry can't supply a diagram, its morning and I'm off to work, will check back tonight if you need a diagram.
Tom.... 