[Rex Anderson]

I worked as a recording and mastering engineer for over 3 decades.

Unless you need to be able to record to both devices at the same time, the best way to handle dealing with your two tape decks is to just move the cables when you need to.

You can't listen to both at the same time right?

A simple patch bay or even just "Y" cables would allow you to feed your source to both tape decks to be able to record on them at the same time. You could swap patch cables from the outputs of each tape deck to the monitor input on your preamp if you are short on line level inputs on your preamp.

When I worked as a mastering engineer, I spent a lot of time moving cables to keep the audio path as clean as possible. Hard wire, home run, whatever term you want to use, just forget switch boxes and patch bays. Move the cables to do what you need.