In my opinion you only need to monitor the house battery bank for capacity removed/returned, temperature, etc.
In the case of start-only start batteries
it is sufficient to monitor the voltage (digital resolution to 50mV). One notes the start battery voltage at times when the PathMaker is not engaged. You should set up the PathMaker's "high voltage" disconnect setting to not connect when the house battery is being charged much above above float voltage (say, set the HV disconnect to 14.0 or 14.1V) merely to aviod unnecessarily gassing the start battery which should always be "full", in essence.
Revisit your philosophy in having a separate windlass
battery, here's why: If you have a high-output alternator
(and you should have at least a 100A capability for cruising with a deep-discharge house bank of any consequence) it is a "waste" to not be able to deliver that amount of current
during the time that you use the windlass so that you achieve maximum rate of retrieval for the anchor
when you need to. To do that you need to have the 14.4V or so available in an "accept" mode of charge. The higher the voltage the faster the rate of retreival with the windlass. In addiiton, there will be less likelyhood of blowing the windlass breaker for heavy loads (all things being equal).
Now to deliver sufficient current
to the windlass from a high-output alternator
you need big cables
to the windlass. If you have sufficiently large cables
to the windlass why do you need a separate battery? In fact, for boats around 50ft or less it is difficult to rationalize the weight of a separate battery over the weight of the cable to do a proper job of delivering current. In addition, the weight of the cable is distributed better over the length to the bow whereas the weight of the separate battery adds to the rocking moment of inertia of the boat
(and, therefore, detracts from the amount of chain that you can carry up forward).
As a final note, if you desire to recharge a deep-discharged windlass battery properly you need to have sufficient current at a sufficiently high acceptance voltage (14.4 V or more at lower temps) which REQUIRES large cable anyway...so, again, why have a separate windlass battery? Move it back to your house bank for added capacity.