I have kept and bred both fish before. I have kept both species in tanks together before successfully until one started to breed. Which 9 times out of 10 is the Convicts. They will breed like rabbits. Once they lay eggs nothing will get near them in the tank. I have had convicts attack Dovis before and the Dovi was nearly a foot in length. The reason I say convicts also is they grow faster reaching sexual maturity at a much smaller size then the Jacks. Jacks will get larger then the convicts but are no match for a mated pair of convicts. I dont deal with South and Central American fish anymore personally, I have been breeding African Cichlids for years now. Soon you will have more Convicts then you will know what to do with.
I have a friend that breeds Electric Blue Jack Dempseys beautiful fish.
If you ever want to try your luck with African Cichlids give me a holler and I can hook you up with some nice fish cheap.
African Cichlids are more colorful and active then their new world counterparts.