How exactly are breeders using the term "incompatible" in reference to specific lines? As a geneticist I know of two uses for it.

So, in terms of ball morphs just to keep it simple I will use the VPI axanthic and Jolliff axanthic lines.

Both are phenotypic axanthic

So, if I cross a VPI to a Jolliff do I get:

1) A bunch of normal looking offspring (that are in essence double het for both VPI and Jolliff.)? In this case incompatible would mean that the two lines are likely distinct genetic loci so even though there are 2 copies of an axanthic-type gene in each of the offspring the offspring are normal appearing because the 2 axanthic-type genes are are at different locations and therefore each is complimented by the normal WT copy at the same location.

2) No viable offspring? In this case incompatible would quite literally mean the 2 different mutations are not compatible when present in the same animal as single copies leading to the death of the offspring