one form, two theme functions--possible?

I have a custom node module, which defines two different node types via hook_node_info(). The difference between the two node types is very slight, which makes it practical to share all the module hooks and helper functions.

I have a form function with a companion theme function. I would like to have two separate themes functions, one for each node type. Is there a way to do this... perhaps defining the theme before returning the $form array at the end of hook_form()?

The alternative (which is perfectly acceptable) is to create a condition in the theme function, but I wanted to take this opportunity to research the two theme method (if possible.)

All comments welcome.