I vote for each of you to carry your own can of pepper spray. As Doug mentioned, there will likely be times when the two of you are separated, if only for a short time (latrine chores, gathering firewood, somebody getting water from the stream, etc.)
There are some bear researchers that routinely carry two large canisters when they hike in to study bears. In the event that they need to employ one on the way in, they will have a fresh one for the hike back out.
Keep in mind that you can't take a can of bear spray on a commercial flight. You will need to buy it once you get to MT, which leaves you little time to figure out the safety mechanism, etc. If you are driving, no problem.
The small (10 oz.) canister lasts only about 7 seconds, which goes by pretty fast when you're hyper and scared. It's probably best to fire a 2-second first blast, then follow that up with 1-2 second bursts if needed. Conserve when you can, since it might be that you'll need some the next day. However, I'll bet that neither of you will have an occasion to use it at all. If that's the case, it would make a nice gift to some forest service employee, or someone at the store where you bought it.
Be sure that the canister is carried in a place where you can grab it, release the safety, and be ready to fire it within maybe 2 seconds. So a water bottle pocket isn't a good place. I gave up on the Counter Assault holster, as it took me too long to get the canister into firing position. For several years I had it on my sternum strap for quick access. Then in 2011, I finally met a mama griz on a trail in Glacier, and it seemed to take 3-5 seconds to grab the bear spray. So now, I have it attached to the left shoulder strap with quick release Velcro (I'm right handed). It's pretty fast access, and out of the way. Here's a photo...