Electrical question: 2 fuses operate together in parallel?

You already verified that all of it will fun on 15 amps, so why not get rid of the 2nd fuse, and change the remaining one to a 20 amp, and be done w/ it.
Or seperate them into 2 15 amp circuits and be done w/ it.
The last thing I would do is letting some unknown electric wannabees in there and really screw things up. You sound perfectly capable of doing this very simple little job yourself. And besides that, warranty probably won't cover it anyway because they usually won't fix what ain't really broke.