calls a drawing that can be interpreted in (usually) two different ways?

Lucy,
I don't know how far back the practice originated but certainly in Victorian times, gentlemen (well supposedly) would often show "Butterflies" to their male friends. These were painted on transparent butterfly shaped paper to make pretty patterns, but when the wings were folded over each other they made another picture, usually pornographic. I have seen more modern examples of triptych or polyptych (to use a word I learnt here recently) art, sometimes used in advertising flyers, which make one picture folded and a completely different one unfolded. I'm not sure if the phrase "ambiguous" would apply to this type of art as each picture is specific (and in some cases, explicit ).

Rod