RFC 2046 (http://www.ietf.org/rfc/rfc2046.txt) in section
5.1.2 (Handling Nested Messages and Multiparts) defines how to send a message in multiple formats:

" In general, user agents that compose "multipart/alternative" entities
must place the body parts in increasing order of preference, that is,
with the preferred format last. For fancy text, the sending user
agent should put the plainest format first and the richest format
last. Receiving user agents should pick and display the last format
they are capable of displaying. In the case where one of the
alternatives is itself of type "multipart" and contains unrecognized
sub-parts, the user agent may choose either to show that alternative,
an earlier alternative, or both."

So I´m 100% sure it CAN be done, however you may find mail clients that do not process the message correctly. But it´s the program fault, not a message problem.