I’m able to record channels 1-16 using Q-Drive “Multitrack” on to tracks 1-16 on my Q-PAC. However, how do you route channels 17 and 18 to record on track 17 & 18?
I could be wrong, but I don’t think you can. 17 and 18 on Qu Drive recordings are reserved for the LR mix.
You are wrong Roy 
Just go into the USB patch on the I/O setup, and patch them (and any others) from where you want them
LOL. No change there then 
But surely if the OP has assigned channels 1-16 to tracks 1-16, and tracks 17 and 18 are reserved for L & R, he cannot assign channels 17 and 18 to any tracks as 18 is the maximum. He can of course remove any channels from tracks 1-16 to be replaced by channels 17 - 32.
Mark is correct, 17-18 can be re-patched to wherever you like, the only limitation is the maximum track count of 18 that you can physically record using Q-Drive. 17-18’s default is to record the L&R, but I have mine set to record channels 17 and 18.
I just said that