Once they got lost you can train them:

Antioch
Canons of the Holy Sepulcher require castle Armoury
Marshall of the Hospitallers require kings stables

Jerusalem
Marshall of the Templars require kings stables
Constable of Jerusalem require kings stables
Unfortunately the games broken b/c Jerusalem can't build a kings stable in the vanilla game. You can however retrain them.

If you let the AI do the recruiting you can have more than one. It is a bug. Apparently the training of the AI doesn't count as recruiting.