W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave.
Isn't it possible to send two incident waves down an xline with
different frequences, and produce two different standing waves by having
some multiplicative relationship between the two incident waves and the
xline length?

Sure, it's possible but one wonders about the application.

2. Matched impedances give ideal power transfer; mismatched
impedances give high SWR and reduced power transfer.

A middle ground - Conjugately matched impedances give ideal power
transfer in the presence of high SWR. A feedline doesn't have to
be flat to be "matched". All that is required is that maximum
available power (actually energy) be transferred.
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73, Cecil http://www.qsl.net/w5dxp