I would just tie both batteries together.
That way, you have less of a percentage of draw from the batteries & they last longer.
Also, you will most likely have the engine running, so the alternator can charge for recovery.
If the engine is not running, you darn well want both of those batteries to power the winch. You would have to put a switch to engage it & the solenoid itself tales precious electricity when you may need it most.
I have been wiring up boats this way for years & have better results with a large bank as compared to 2 - 1/2 sized banks. 1/2 the discharge depth (percent of battery) & 1/2 the cycles.

Ed