First off, you’d be smart to get your hands on the job prints (if still available) and copy them, or a comparable duplex set from that era.
Yes it can be done...it happens all the time. You say “master and slave” cars...this is a US MP1230 processor based controller?
There is a car 1 and car 2 designation if it’s a 1230 duplex, but either car can be a simplex even with fire service.
There is a bit more to converting the 1230 duplex to a simplex (good luck there), but job prints will help on the controller side.

If it’s a standard R10 relay based control, again, either car can be a simplex, and you’ll need correct prints for the job to separate or remove duplex interconnections.