Most likely the hour meters are working, but not displaying. This happens on and off to me also. The Volvo
tach displays are notorious for this. What happens is that when the batteries are charged and the alternator
field voltage drops, the meters (and often the tachs) go out or read gibberish. If you turn on an electrical
load, they will come back on as the alternator
kicks back in.
So, you have three choices: 1. live with it as is. 2. turn on a load while underway. 3. install an aftermarket hour meter.
As a consolation, you now know which of your alternators is the dominant one, since the tach of the "weaker" alternator will cut out first!
BTW, what cat do you own? I have a Manta