16x20 would take four sheets of 8x10. Two 8x10s would be either a 10x16 or an 8x20 depending on which way you turned the paper.

Did you do that in one easel? Interested in how you kept the two prints aligned and the paper flat. This could have uses. I'd be inclined to print those on one 16x20 sheet since I can do so, but this would also allow, say, 14x22 or 11x28 panoramas on two sheets of 11x14, or correspondingly larger sizes using 16x20.