Re: Adding a DPDT switch to the sense lines ?
Yes it can be done.
In fact, in one sense it must be done for if you leave the sense lines connected to your charged battery it's going to tell the controller that the battery is charged and it won't send much current to a different battery now connected to the output lines.
At the very least you need a SPST on the (+) of the sense lines to disconnect them from your regular battery. The controller will still function, relying on feedback from the charge leads to give it Voltage info on the battery. The sense lines are more accurate in that they don't carry the charge current so the V is the same at both ends, as it were.
You could use a DPDT switch to change both the (+) and (-) from the 'main' battery to the 'accessory' battery and get an accurate reading on it. Just remember you have to switch both the sense lines and the charge lines. This could be built up to a level of complexity that would switch both simultaneously, but that's somewhat unnecessary.
Be sure the 'temporary' connections to other batteries are as solid as 'permanent' ones or else you won't get good charging.
Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.
Ohm's Law: Amps = Volts / Ohms
Power Formula: Watts = Volts * Amps