Bill, yes, you can run another mic through the arranger keyboard, and use the onboard harmonizer. Take a look at: https://www.psrtutorial.com/music/articles/11-VHPrimer.html where Dan provided one of the best and most comprehensive instructional information on using Yamaha's harmonizers. He even provided a few demos and shows you how to avoid dropout.

Keep in mind, though, you will still need a mixer to use 2 mics. You can only route one mic through the keyboard, which will be outputted via your main out from the keyboard, while the TCH has it's own outputs, which will then be fed through the mixer in conjunction with the keyboard, and all terminating in the Bose L1 Compact. On some of the larger jobs, I used a pair of L1 Compacts, which provided excellent coverage for up to 250 ppl. For jobs larger than that, I fell back on my Bose L1 PAS, which was incredible, both sound and coverage area.

Hope this helps, but if you need additional assistance with the hookup, you have my telephone number - it's still the same.

Gary