You're right about not supporting Array values in the field property, although it might be worth exploring because the question does come up every now and then.

We generally recommend one of two approaches:

1. Option groups. This is better if the second field is dependent on the first. Example here: Multi-level field selector #747 (comment).
2. Delimiter-separating the field string value (e.g. with dots like "val1.val2" or vertical bars like "val1|val2"). This is better if the two dropdowns are truly independent of each other. Typically this involves something like props.value.split(delimiter) before rendering, and then updating the value with something like props.handleOnChange([value1, value2].join(delimiter)).

Case 1 is the most common, but if you need an example of Case 2 I can throw one together or maybe find an old one.