Two males, the big male is dominating the smaller one. yes dominant male
cichlids will act like that with subordinate males. challenging and
flashing, and chasing keeping the subordinate male in it's place.
It's really not going to work long term, he's going to hinder the smaller one from eating enough to grow taking most of the food for himself to stay the dominant fish, so they will always have a size difference if kept together. If you wanted to balance them out in size, they'd need to be separated and the small one given more often, smaller feedings, that equals more food consumed each day so he can put on body mass and grow, while the larger one goes on a diet.
Jack Dempseys run in a temperature range of 72F to 86F, with 78F being the middle ground if you run the big one
cooler, say 74F, his metabolism will slow down ( so does digestion) and make him want to eat less often. And if you run the smaller one warmer, say 82-83F, his metabolism will speed up (also digestion) and make him want to eat more. It helps speed along or slow down growth doing this short term, like a few months to allow smaller fish to catch up. The big one processes food slowly, the small one processes food quickly and starts to catch up in size.
When it catches up in size you could try them again see if it changes the dynamic at all. but usually only one male is going to be in charge and assert it's dominance over the weaker/smaller one, but they can come to an agreement without a female present that they are equals in size and strength and with enough food available, they can learn to coexist also.
No guarantee that works out though, it may just change who's picking on who, or remain the same dynamic, but it could work.
If you plan was to have a male and female, then certainly this isn't going to work.
