Hi Colin - There isn't a way to do this at present, sorry. The best thing to do in this situation is to separate the overlapping sections slightly (best done using the Reshape Route tool with the Auto-Plot switch turned off), so that you can then more easily select the correct section. Longer term we hope to develop a new tool to automaticlaly seperates overlapping sections of routes - this is on our Feature Requests list (number 12).
A few times I've created out and back routes and then realised I need to reroute small sections. Unfortunately becasue the routes are exactly the same it seems very hit and miss as to whether the start and finish points end up on the 'out' or 'back' section of the route - which obviously dramatically affects the amount of the route that I have to replot. Is there a way of forcing the start/finish points to both be on the out or back section rather than one on each?