They are on the same server and document root. They are "sister" domains-- it looks like this:
/home/account/public_html/alpha
/home/account/public_html/bravo
The hosting company redirects
www.alpha.com to /home/account/public_html/alpha and redirects
www.bravo.com to /home/account/public_html/bravo.
I've tried RewriteRule ^(.+)/(.+)$ /../bravo/codedir/test/$2 [NC,QSA,L], the idea being I move up one level in the structure and then down to bravo/codedir/test, but that did not work. (I'm not at my computer so I'm afraid I can't give you more details on what that did until later)
I also tried the [P] directive but that broke the redirect entirely-- presumably because the mod_proxy is not there(?)... but yeah I'd just as soon do it without the proxy directive if possible.
Thank you Richardk for your help!