A step-thru folding bike is my recommendation. Extra long seatposts and adjustable steering tube gives you great size adjustments. You likely will need a second seatpost and saddle if the long one drags the ground when setup for the smaller person, but this is no big deal and far cheaper than two bikes. Takes up no extra storage space either. Just be sure the steering tube is telescopic. Use a sharpie or paint pen to mark each rider's preferred adjustment points.
Something like a Dahon Ciao style should work although I don't believe it has a telescoping stem/steer tube. Many bikes this style do.
