How to vent a 2nd clothes dryer?
A dryer moves a lot of air, and requires a dedicated 4" vent.  For two dryers to operate at the same time, you would need a 6" vent.  Just as air conditioning ducting gets smaller as it splits off, the two outgoing vents will need to join into a larger vent.

The area of a 4" vent is (πr^2 = 3.14 * 2 * 2 = ) 12.6 sq in.  A 6" vent is 28.3 sq in. so it can handle the two 4" vents (you wouldn't have to double the vent size to 8"...).

Combine this with the need for an additional electric run, and your spare dryer is turning into a big expense!