Is it possible to just buy 2 tires only (instead of all 4)?
Yes, as long as your mystery vehicle is not equipped with AWD.
Bear in mind that if you buy ony two tires, a reputable tire shop will only install them on the rear wheels, as per the protocol of all of the major tire manufacturers.
Is it worth it?
It all depends on whether you are a gambler by nature or not. You might drive for years without having tires damaged by a nail or glass or other road debris, but then again, you might have more than one incident of this nature in only one year. Nobody can predict what you might run into or over.
You should be aware that Costco includes a Road Hazard Warranty in the price of their tires. The prices tend to be good to begin with. Once you factor in the Road Hazard coverage, free lifetime puncture repairs, free lifetime balancing, and free lifetime rotations, the price is even better.
I am not suggesting that you join Costco just for this, but you might want to consider a membership for one year and scoop up all of the other bargains on food, paper supplies, motor oil, car batteries, etc, in addition to their Michelin and BF Goodrich tires.