First off, don't let anything said here dissuade you from what sounds like a wonderful adventure. That said, as with any adventure, hope for the best but prepare for the worst.
I live in southern California, and so my experience with subarctic climate is practically nil, but if I were in your position, here's what I would be investigating:
Replacing the oil with an oil of proper viscosity for the temperatures you anticipate.
Does your vehicle have a block heater? Should it?
Should you alter the ratio of anti-freeze to water?
Should you replace the windshield washer fluid with de-icing fluid? Certainly a windshield scraper would be handy.
Both vehicles should have tire chains.
Maybe both vehicles should have studded tires.
Is there reliable cell phone service all along your route? Have a list of local emergency assistance numbers.
How well are the roads along your route maintained? How often plowed? If you can, get some insight from someone in Yellowknife or elsewhere in the area.
Does the navigation system have maps for the area in which you intend to travel?
Have a back-up nav system. Even if it's just paper maps and a handheld GPS. You don't want to get lost.
Is fuel available at intervals coinciding with the range of the vehicle? Do you need to carry extra fuel?
You should carry water and food, flashlights, extra batteries, etc. just in case.
Without knowing any details about your route, I'd say the suggestions of a shovel and tow straps are good ones. Besides the tow straps, you should have shackles to attach the straps to the vehicles. Practice mounting the chains and practice with the tow straps under ideal conditions, before you get into trouble, so that should the need arise your stress level is lessened. Know how to remove the access covers for the recovery points on the vehicle. If you can get recovery training from a knowledgable source, that would be ideal. A pair of walkie-talkies or other two-way radios are always nice to communicate between vehicles. If bad weather closes in on you, you should have pre-planned places to stop and wait it out - places where there are services available, of course. Check in with someone at home every day, and let them know whom to contact should you fail to check in.
If you are not familiar with driving in snow and ice conditions, don't miss any opportunity to practice before you set off on your expedition. It sounds like fun!
That's all I can think of off the top of my head.
