Covering the tires does little good. Tires will dry rot due to exposure to the air. Not using tires is very hard on them. Tires are designed to be used, develop heat and then chemicals within the tire work to the surface to protect the tires.
I almost learned this lesson the hard way. I had a spare tire that had been under the bed of my pickup for 4 years. I decided to put it into rotation along with a newly purchased tire on the other side of the axle. Everything was fine. I spent a couple of weeks getting ready for my trip and then noticed the weight on the tire had caused severe cracks to form. I would be very cautious about tires that had been sitting for 2 years.