210 FSH Battery Charging question

[I know You know]

I use a small clip on battery charger to keep the batteries on my Jet Skis topped off, I am now also using it on my new 210 FSH. Can I turn my on board battery yellow emergency parallel switch to ON and charge both batteries at the same time by clipping the chargers leads to one of the batteries, and if so which one of the two.

 

[highanddryinco]

(Working off the assumption that the 210 setup is the same as the 190...)

You can, but I would probably just hook my clips to the starting battery and let the VSR (Voltage Sensing Relay) do the work for you. When the starting battery reaches 13.5V the VSR will let the charging current flow past it and on to the house battery.
That's the very simplistic explanation. Depending on your clip on charger type and rating, it may be tapering off its output substantially as the starting battery gets close to 13.5V. I'm thinking you'll be fine though as it sounds like you're mostly looking for battery maintaining.