Outboard battery connection and charging issue

[Rum_Pirate]

Two 85hp 2-stroke Yamaha outboards installed originally with two separate batteries.

They were installed with separate battery isolator switches.

The negative terminals were connected.
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Son had a chap do some work.

Now one isolator switch has a positive running to it and no other connection, i.e. it is useless!

The batteries are now wired in parallel (fortunately not in series).

There is no12 volt Dual Battery Switch - One Two or Both - Marine Battery Switch Isolator

Surely it is not correct for two outboard alternators to be producing a charge and send the current to the same battery the same battery.
Does one not produce maybe a higher voltage and then overpowers wrecks (?) the other one?

Is this normal practice?

What problems do I face?

 

[gordmac]

Not unknown when a chap does some work you need to spend more time sorting the mess than it would have taken you to do the job right in the first place!
If you have separate systems for each engine just have some way of connecting one to the other incase one goes flat. I made up a short jump lead for that purpose on a previous boat. On that boat one battery did everything including start both engines, the only thing connected to the other battery was one alternator!
What do you do about domestic and lights?

 

[Rum_Pirate]

Not unknown when a chap does some work you need to spend more time sorting the mess than it would have taken you to do the job right in the first place!
If you have separate systems for each engine just have some way of connecting one to the other incase one goes flat. I made up a short jump lead for that purpose on a previous boat. On that boat one battery did everything including start both engines, the only thing connected to the other battery was one alternator!
What do you do about domestic and lights?

Ideally I wanted two separate isolated installations, one for each engine.

here we are quickly in open ocean waters. Don't have a Coast Guard like UK or USA - basically you are on your own for a while at least, hence two engines.
The original installer had the two batteries connected at the negative terminals.


As to domestic and light, well the boat is :

An early 90's - 23'0" Mako, centre console with T-top, two 85hp 2-stroke Yamaha outboards with electric trim and instruments, tow bilge pumps (each supposed to be to one engine), a VKF radio.

 

[gordmac]

No problem connecting the battery earths together as long as the + aren't, this allows you to jump one battery off the other by connecting the + together if one is too flat to start. Obviously have the other engine running to charge when doing this. This is what I did using a short jump lead, other option would be by a cut off switch. A cut off switch in each live is a good idea. The electric stuff can come of one battery or some off each, I would go with the former to reduce the risk of flattening both batteries. No need to complicate things with fancy dual switches!