A lot of Masters racers do this, I used to regularly do Masters & 1,2,Pro. In my experience the schedule really determines how well this works. You want the races to be either far apart (>2 hrs) or close together (< 30min). In the first case you get off the bike, take a break, maby a bit of massage & something to eat. Then start your regular warmup routine for the next race. In the second case just keep riding, stay warm, maby an energy gel & fluids. You need to learn what works for you.
In between these two times for me usually caused problems and I would struggle early in the second race. Never really felt overextended doing this and IMO it greatly improved fitness.