Grounding issue, engines grounded to each other, clean tight terminals on studs. Clean tight connections on all battery grounds.

There is a separate ground going from each battery to each block Except he connected the 2 blocks also together grounding to each other for a additional ground. Is this NG???

post engine serial numbers and what ignition set up - and is the ignition set up 100 % stock ?

If this is a T Bolt 4 stock Merc set up --- I would change the cap and rotor and THE IGNITION SENSOR UNDER the rotor. And the ignition coil.

The new IMPROVED ignition sensor has its own separate ground strap for the past x amount of years to be mounted on the backside engine grounds. And is fully potted against corrosion.

This possible could be part of the problem as my thinking that darn sensor under the rotor. And the ignition coil

Just a WAG as this because WITHOUT eyes / hands / ears / Volt meter / and other forms of testing not performed on this app it boils down to a WAG.

Never seen a TB-4 on a MPI (454 or 502) all have been EST

See post #24 on.

Bob me either on a big block MPI Merc engines but will add --- T bolt V was used on 4.3 and 5.7 MPI and EFI engines thou out using Mefi controllers I sure have seen a lot of those as well..

Just to reference back ---- in his post reply back from mine --- way back in the thread -- he said these were carb apps. I did not know at that time what set up he had either.

Stock Merc Carb apps as I never seen EST systems installed on those apps.

Thanks SB and in post # 26 he says Hardin Marine distributors installed.

A lot of times, I do not read ALL the posts because of time - I come in from break to grab some AC or look up parts and jump up here quickly., Then get back at it.

So these are MAX Volt set ups for Ignition set ups ???

Yes
Hardin Marine - "Run Ready" Max Volt Ignition Package - Chevy V8

35 I dont know if the grounding the blocks together could have any effect like that or . Not sure how he tested the batteries or if he replaced the batteries but when using a load tester, make sure NO other wires are connected to the batteries. And I see no reason to connect the 2 blocks together. I still think this is Battery/ ground related.
Chris

Nordic, you have a bad battery. All battery's should be grounded to the block otherwise they dont charge as well. 1 battery will get charged pretty good and 1 will not charge as well over time. This can /will ruin both batteries over time. Many Many boats just have a jumper ground to the second battery and that is just not best practice. I had that exact problem on a single engine dual battery set up.
Chris.