A few tandem manufacturers have Large/Small as one of their configurations. I have a 23"/18" Co-Motion that I ride with my 9 year old daughter. I'm 1.81 m tall with 88 cm inseam (5' 11" and 34.7 in), and she is 1.40 m tall.

Two things worth remembering:
– The pilot cockpit need to be sized like for a single, but keep at least 10 cm of seatpost exposed because of the stoker stem.
– The stoker doesn't need to reach the ground because it's only the captain that puts his (her) feet on the ground at intersections. So while an 18" single frame would not yet fit my daughter, it's perfect for the stoker compartment.