There is a company that makes high quality signal routers - Mapletree Audio. You can have them configure one however you want it or you can go with the standard 3 in & 3 out.
This is the best solution IMO if you want to compare phono preamps. You can have both hooked up and you can switch at will. I personally prefer short term switching because I know from experience that my system can sound better or worse on different days without changing anything (I think the difference lies within myself as opposed to external forces). I will usually pick a single track, play the track all the way through, and then play it again through the other component. I also like to play a short section, maybe 10 seconds, of a track that represents a particular sonic characteristic. Being able to rapidly switch facilitates a good comparison. I will do this over a couple weeks at different times because I know that my hearing acuity is not always the same. I'm distrustful of changing out a piece of gear and listening for a few weeks because researchers have demonstrated that audio memory is very poor after a short period. It's a golden opportunity for expectation bias and confirmation bias to set in.