A "combiner" is usually a device that combines batteries.
Can you start again with the problem description....
a. for the first engine to start the tacho works immediately on starting...correct? test and confirm please by starting the test with both stopped, start one, observe & record tacho, stop engine and then start the other, observe & record tacho, stop engine
If engines are run independently the tachs work fine.
b. the second engine to start, whichever it is, tacho doesnt work until the 2nd engine rpm is over approx 1500 rpm...correct? test and confirm please by starting the test with both stopped, start one, observe & record tacho, then start the other, observe & record tacho, then increase speed on second engine until tacho kicks in, record tacho for both engines at this point. stop both, repeat but starting with opposite side first
The first engine to start always has tach indication. Raising the 2nd engine to >1500 rpm causes the 2nd tach to kick in. Lowering the rpm <1500 causes the tach to kick out. This is repeatable with both engines when they are the 2nd engine started.
or, describe the problem again in your own way !!
I think what is happening is that you start one, it's generator starts generating, you can tell it has done so because you can see rpm on the tacho. Once it has started generating the combiner senses the generator voltage on one side and "combines" the batteries so that both banks are now being charged from the only running generator (the one on the first engine to be started). You then start the second engine, it's generator has to get up to the voltage of the first generator before it can start delivering to it's battery, because it's battery load is already at an elevated voltage from the first engine's generator through the "combiner" this does not happen until you speed the engine up somewhat, after that it kicks in and you get rpm indication
The Promariner combiner I have is designed to share the load between the two alternators. That is its primary function is to increase charging capability.
I will know better once you give the result from the above described test routine, the final question, if it is happening as I have described above.... when you slow down the second engine to tick over after the gen has kicked in does it continue to give rpm indication ?
If you slow below 1500 rpm it drops out. Above 1500 it kicks in. When the tacks are indicating they have accurate rpm readings