I install a automatic acr and a switch. That acr sent the power to the battery that needed.
you can view it here: https://www.bluesea.com/products/7649/Mini_Add-A-Battery_Kit_-_65A
How to install: https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/instructions/980014350.pdf
If you're not sure you can send a email to them and they are very fast to answer.
Hope this help