Is there a way to control multiple external parameters with the one CV Out on my Theremini?

Yes. 

If you first connect the TS cable from the CV OUT on the Theremini to a mult input, you can then send that signal to various different CV Input parameters. 

For example, if you connect the multed signal from the Theremini’s CV OUT to both the 1V/OCT & VCF CUTOFF inputs of a Mother-32, you can control both the pitch & filter simultaneously with the Theremini by a wave of your hand.