My house has an outside light over the garage door which doesn't work.
My goal is to replace it with an automatic flood light. There is a
light switch plate inside the garage on the same wall that has two
switches, one for the lights inside the garage and one presumably for
the outside one.

I have taken the light fixture off and checked the wires but don't see
any voltage there no matter what position the switch is in. My next
step is to take the switch plate apart and check for voltage there.
Would I be right to assume both switches should be on the same circuit?
What other troubleshooting tips do you have?