Having a voltage issues on one engine. Between myself and asking advice from my dad who has experience rigging boat”s ,also asking friends/ mechanics at a point where I am lost on what to do next.

98 35 fountain lightning/ex. 502 Blue HP500 carb thunderbolt 5. Setup with 2 batteries one for each engine, replaced battery switches so i know starboard runs off #1 and port off #2.Port engine is having a voltage issue where alternator is shooting over 14.5volts through system when engine revs up on plane over 2500rpm coils heating up losing power ,epoxy coil that is mounted on its side , I know oil coils need to be upright. At idle seems fine 14.5v steady 14-14.2 when under load from trim pumps. Checked grounds cleaned them ,re did anything that looked skeptical.Boat has silent choice Gil risers nightmare to look at anything back of engine on them, so took exhaust off to get a good look. Took bracket off back of engine where ignition module and knock sensor module are located. Cleaned everything and was hopeful. Thought bad ground , cleaned made sure they are good. Ground bus located on transom in middle behind engines. 4 grounds on bus , 1 battery from each engine and 1 individual wire running from bell housings of each engine to bus. All changed/ cleaned .Saw corrosion on ignition module/ knock sensor module that looked like could have been an issue. Problem still there. Tried another thunderbolt 5 module I have as spare issue still there.Took it out this past week, issue still lingering. Starboard engine runs fine same setup. Originally before tearing everything apart thought it was alternator. Took alternator off engine with issue to a reputable auto electric shop. Watched him test it under load on machine voltage stayed steady at 14.5 down to 14.3 under load. So assumed it was good. After 3 weeks tearing everything apart came to the conclusion it was my alternator or battery, possibly orange wire or ground on alternator, checked them all everything good . Swapped battery at dock fired up reved to 3000-4000 ohm meter connected to battery still showed high voltage.Ran a separate ground from alternator to block as I noticed the ground wire from alternator runs into the wiring harness , issue still there. So trouble shooting I swapped alternators from engine with issue to engine that had saved me from a tow or paddle home. Hallelujah, issue went from port to starboard didn’t leave dock just ran it and reved it up in neutral , clearly saw high voltage on the engine that had been working fine with alternator that was on bad engine as high at 17v over 2000rpm so I assumed I found problem with the issue following the alternator and the test at electrical shop was wrong . Put new alt on today took it out and instead off seeing voltage of 15v-17v saw 14.8v-14.9 steady with rpm over 2500-3000rpm high enough to heat coil up and cause loss of power. Shut it down stopped before I cooked another coil. Slow down off plane voltage drops to 14.5v. Fountain seemed to wire engines together under dash with alarms , changed that pulled dash apart nothing alarming considering it’s a 98, so each engine alarms are separate now. If I disconnect port alternator orange wire (engine with issue) and run the switches on both. Changed them when engines are off. Boat runs fine as it should no high voltage.Essentially running off one alternator with no isolator. When I bought the boat , the trim pumps were wired to run solely from one engine. The engine that is giving me issues (port) ran both trim tab pumps along with both trim pumps for drives. Changed it so each engine runs its own drive pump and one engine runs both trim pumps (port). Would of had each engine run its own trim tab pump but the way it’s wired under seat and breaker panel didn’t want to cause more work. Before I did that I would see a huge voltage drop when I trimmed both drives with “both” button on dash. At a point where tearing everything out and re rigging seems inevitable.The only grounds I have not physically seen on boat are located behind the fibreglass panel located behind back bench that is directly under your nose when you open hatch. Any ideas on what to do next or test without pulling engines would be appreciated. Long post but tried to provide detail , this issue has ruined this boating season as I am in Canada and its short. Any input or ideas to test I appreciate.

Are you using Merc Alternators?? and Merc coils???
Sounds to me like the voltage regulators in the alternators you using are set too high