With both engines seemingly being affected simultaneously, I would suspect something external to the engines themselves.
When you say "recently they don't get above 1850 rpm" did that happen gradually or all of a sudden?
Have you changed anything like added weight or components to the boat that might give additional drag or load? "The props are clean" but do the shafts turn freely? When were the transmissions last serviced?
Fuel quality. How old is the fuel load and are you certain of the supplier? Did you refuel at a different fuel dock recently? Are the tanks clean and free of algae? Bugs will clog a new filter in mere heartbeats.
You could take fuel pressure readings as a start to engine diagnosis. But, even with good pressure, you still need proper flow for achieving full power so a fuel return spill back check will tell you if you have any restriction in the lines that shouldn't be there. Any competent diesel mechanic can do these tests for you if you don't know how.
One other simple check would be to make sure that your turbo wheels are clean and rotate freely and that there are no exhaust leaks or air leaks in the ducting from the compressors to the intercoolers or blowers.
good luck.
Glen