I spent 3 years or more dedicating several hundred gallons of tanks into breeding Blue Dempseys... beautiful fish and a fun project...
It's been pretty well proven that Blue x Blue is a lethal combination. I've done this with 3 different males and three different females. Every time the eggs were fertilized and hatched, but the fry died in less than a months time. All three males and two of three females were bred with non Blue Dempseys and their offspring did fine (the other female died before being bred elsewhere). I've also talked to several others with similar experiences and have not once heard of anyone who raised Blue x Blue offspring more than 6 weeks.
I personally strongly supported the theory that blue Dempseys were weakened due to excessive inbreeding. While I do still believe that inbreedign depression is a fact to consider, both through learning more about genetics as well as through practical experience breedign Blues, I no longer believe inbreeding is the cause of weakened Blues and blieve the mutated gene causes weakened Blue Dempseys.
I did find using "better quality" breeders yielded "better quality" offspring. But I do not believe that even many generations of doing this even coupled with extensive outcrossing, will erase all of the ill effects seen in Blue Dempseys.
It was highly suspected that Het Blue (BGJD) x Het Blue (BGJD) yielded better offspring. But I have seen zero evidence of this in my own breeding projects...
I suggest seeking out Het Blues (or split gene as you have called them) as well as creatign your own. Then use trial and error to determine what fish make the best mates for you.
You pairs of dempseys will do fine in tanks such as a 40 breeder (3' x 1.5') but full grown adult pairs will do much better in larger tanks (such as 4' x 1.5').
I had the absolute best success raising fry and grow outs in smaller tanks that were plumbed into larer systems. This allowed me to have very high water quality even while keeping many fish in a somewhat small tank.