I am not understanding what the reason is behind your setup. Usually, 2 batteries which are designated as starter and leisure use multiple switches (3) with a cross over functionality. The installation of a auto sensing split charge relay, allows for the leisure to be charged when underway. Beauty of the above, allows the leisure to be drained when at anchor etc, and will leave you with a full charged starter battery when you head home. The cross over option, allows you to start the engine via the leisure battery in a flat starter situation. Great redundancy. The 16mm cable should be sized to allow starting of the engine from 5m away, not charging only..... The model of outboard, will determine how big that cable really needs to be. Happy to chat through if you'd like. R