Unless your indoor air is already roughly 45-55% RH year-round, you should already be storing those acoustic guitars in their cases with humidifiers, at least during the dry months.

I really like the Oasis humidifiers. Get one for each case, and pick up their hygrometer too to throw in the case so you can keep track of the humidity. If the cases themselves have never been humidified they can absorb a lot of moisture, and it may take a week or two to get the humidity inside the case where you want it. If you're having trouble getting the humidity into a safe range, you may have to double up the humidifiers or make some DIY ones.