Pictus Catfish, Hoplo catfish, Synodontis. Pretty much whichever catfish you ant really. Within reason that can fit in with your tanks stats. I really want to try Brichardi (spelling?) catfish with my Jacks, they are magnificent.
You arent just limited with catfish. It all depends on your Jacks personality, i've had a few Jacks to date, and i've kept my male 8" with rummynose tetras no problems. He did grow up with them i'll add and has always been fine. The only thing i've found i couldnt keep with my Jacks were shrimp. They saw them as food.
What size tank are you keeping your two Jacks in and how big are they themselves? You say fully grown. That could be anywhere between 6-10" depending on their sex.