Someone once mentioned that you could share all the data lines and have separate enable pins so that you could technically use two with only 16 pins, but I have no idea how well / if that even works.
There's no reason for it not to work - this is precisely how the 40x4 displays are configured except that both of its 20x4 displays are in the same glass enclosure. You share the RS line (and the R/W if used) in addition to the data lines. Only the E lines are separate.
Don