Installing two starter batteries

[tarian]

I run a 24v boat . 6 leisure and 2 starter in series the mppt and battery charger connect to a positive bus bar which is also connected to a durite smart controller this then takes the charge for house and starter and feeds the current to which ever battery needs charged
I run from the main feed to my current panel and then run a 24c to 12 volt reducer to a separate fused panel here I wire up any 12v appliances like vhf stereo etc .
Redundancy is not needed I can easily take the leisure feed and put this into my starter to start the engine , or for a really simple solution get a set of jump leads which you can jump from house to starter . It seems you are over complicating a simple solution which is the durite

[SV__Grace]

I like the idea of a Jump Start battery as a backup and commonly use mine for the air compressor feature.

During our electrical refit our guy removed the start battery from the charging system as it gets charged by the alternator when the engine is running and doesn't need more than that.

As a backup, he installed a separate start battery for the generator and re-purposed the battery switch to be able to switch start batteries if needed, and the "Both" setting combines them if needed. If you have a jump start battery that would be your backup.

Both engine and generator have alternators that typically replace the energy used to start them. Not having them joined to the house batteries simplifies my solar/wind/shore power charging system.

My arrangement would only work on a larger boat with an inboard generator, but it makes sense to me for a smaller boat's engine start battery. I haven't seen this config on other boats and I'm happy with my setup.

[NorthCoastJoe]

I would switch from the 24 volt to the 12 volt if I needed to. I had a old VW bus that I converted from 6 to 12 volts. In the process I left the 6 volt starter and had no problems running 12 volts through for years.

Use a break before make battery switch. The short amount of time the starter gets 24 volts will not hurt it. Just be sure the only thing getting the 24 volts is the starter.