Nice job and idea on using the Blue Sea marine quality battery switch. We have been doing this same idea since 2009 here on my four GP24 12VDC Interstate standard issue batteries. My layout is for four batteries (two groups of two each) with each groupd using a four-way Blue Sea Battery and then a third Blue Sea four position switch selecting the two groups. Certainly gives you full control of using one, two, three, four, or all off letting you play games with what batteries you want to use for camping or other things. Each Blue Sea switch position is OFF-BAT1-BOTH-BAT2
My photos here only shows three GP24 batteries being used. I am now thinking upgrade mode since 2009 and will be installing a metal box with two 150AH 12VDC batteries here on the trailer tongue with another group of two each 150AH batteries mounted in the back of my truck bed. This allows me to use my batteries for separate things I am involved with as well as using them all together giving me around 600AHs capacity when camping. The two locations required to spead out the battery weight also.
I was going to use the enclosed optional back cover for my Blue Sea switches like you did but never got them ordered. Looks like a more safer install having the heavy cables ring terminals enclosed and out of harms way. Without the back cover the cables are rather close to the trailer frame but I have not had an issues with them being this way...
my setup from the Off-Road Trailer 12VDC Power Distribution panel to the batteries is shown here.