I use Fender reissue combo amps for backline 99% of the time.
4×10 ‘59 Bassman primarily, the 2nd amp normally being a blackface reverb amp of appropriate power; Twin Reverb, Super Reverb, Vibrolux, or Deluxe Reverb.
The BF reverb amps channels are out-of-phase anyway, so one channel has to agree with the Bassman.
That would take care of it right there, no problem.
The non-reverb channels of the BF RI amps is usually in phase with the Bassman.
The reissue Fenders normally use push on spade connectors for the speakers.
If I had to flip blackface for for technical or aesthetic reasons I’d just pull the connectors and swap them.
Alternately, assign some phase reversing effect to the input I wanted. Sarno Black Box, Mutron, various compressors.
Push come to shove, you can just turn an open back cabinet around and you’re good to go.
So anyway, to answer the question “phase issues?”, sure, once in a while, but phase or polarity is easily handled by amp channels being in opposite phase and the ease of reversing speaker polarity with the normal Fender spade lugs.
If I were expecting to use separate heads and cabinets I’d either pack one extra reverse-wired speaker cable along with the normal pair, or some other variation of adapter; reverse wired male to female phone, or my fave, a couple of banana to phone speaker cables.
I think it’s better to flip speaker polarity than use more stuff in front of the amp if you can.