Don’t take this as berating you I’m just getting straight to the point, it simply does not comply with ABYC, RYA, Transport Canada, or any electrical codes related to ship construction or standards, that’s just a fact and I’m not going to debate that. there are plenty of reasons why, one would be that the battery switch is a weak link in the chain, and if you have a short like that it could very well weld or melt parts of the switch making the switch useless. Do you really want to be undoing a hot, and potentially burning wire with welded posts while getting pushed towards shore, sucked though portlier pass, dent Island devils hole, sookumchuck or any other of the tens of thousands of bad scenarios. Its potentially putting your lives and vessel at risk and a 50$ switch will prevent that. Heck if you don’t want to spend that money I will give you a switch!! (I actually do have some used ones lying around if you want one, no joke).
In reality you should have one battery per engine including the kicker, and there is plenty of reasoning behind that as well, because if you are running a single main engine and a kicker, and the regulator in your alternator fails you could discharge or fry the battery quite easily, then neither engine will work.
there are other regulations that are often overlooked that shouldn’t be, Like you cant have more than 3 connections on a battery terminal, missing adequate fuse protection, not properly mounting batteries to prevent acid from spilling out, undersized wiring, too many devices on a breaker.... the list goes on and on, and I see it nearly daily. And I’ve lost count of how many times I’ve seen bilge pumps wired with marettes, heck one very well known 68' Grenfell at RVYC was wired almost exclusively using marettes.
Again there is a good valid reason for every regulation, and I am not joking when I said for every regulation is written in blood. The marine industy is one of the oldest in the world, there is a reson behind every regualtion and for one to be written it means something went wrong that caused loss of life.