Any two adult discus who are in the mood, assuming the appropriate sex balance, will mate. For Heckel crosses, these are typically one way whereby the female is Heckel. These fish do not have to be raised together, nor do they necessarily have to spend a lot of time together. When I force pair two discus, they'll usually spawn within two weeks. If they don't do it in 14 days, I'll break up the pair and try a different combination.
I'm only aware of a single verified report of a successful Heckel X Heckel cross. That would be the inimitable Herman Chan of Fairy Lake Discus Palace in San Francisco back in the mid-80's.