Have you got an assisted chin up station?
If not, do you have large lifting bands?
2chin ups is tough to build up on without making the movement easier somehow.
If all else fails I would just do 5 sets of 2. Make sure you are having enough rest between sets that you will make both reps. Try to do this as much as possible (daily even). They should get easier and be able to add more onto them.
I'm no pro though! I am just now focusing on building my chin ups from 11 up.