I did the route dual, then solo. At the time it seemed like an excellent way of doing it, insofar that it was a more gradual build-up to the qualifier and as such, a more effective training system.
Do not underestimate how big a leap the solo QXC really is. Whilst agreeing to some extent with the comments of tacpot, I am not sure of the wisdom of throwing a student in at the deep end at any point in their training. If the dual route is treated as ghosted (ie, the instructor is there as a backup, not as an instructor), you can fulfill your wish to check their abilities without the fear that they are being steered the whole way by the right-hand seat.