I think there was some friction between the two, artists temperment
Maybe because they stealing each others art work, could make a person testy.
"artists temperment 'aside'.."
Stealing each others ART work?
Perhaps,,copying of game scenes and layout has been common in the field forever, though still looked down upon. (There are copyright laws also)
Few engravers have the raw talent to sit down and draw up a scene. Both Fugger and RK were good artists, no doubt about that.
It still takes time to draw up a scene/layout. Stacks of pictures, clippings, layouts, a 'morgue' of animal drawings cut from magazines and books was and still is a part of most engravers needs. The computer has taken the process further and made it even easier to make up, size, copy and transfer.
Once an engraver hits on a good game scene you'll often see that repeated in their work over time. Others will copy it, even a scroll style.
The 'Neidner Ballard squirrel' mentioned above by Greibel also shows up on a steel trapdoor style buttplate done by RK.
So who copied who,,either mad at the other? Did one take credit for the little guy when the other did the cutting first. Maybe (probably) it's just a copy of a picture of some other artists drawing from a book. No credit given there.
It's the time saved in the process that is the benefit even if you have the talent for drawing it up youself.
I have copys of Bill MacGraws layouts. All those ducks, Indians, Trap Shooters, Mermaids, etc didn't come from the artists mind. Magazine clippings of ads, currency, stamps, etc.
RK accepting credit for completed engraving projects done by Fugger and others seemed to be the problem.
S&W RegMag engv by Fugger and a Win73 by Krieghoff engravers come to mind.
But not the other way around AFAIK