Get the DB Initiate to accompany you to another follower (Helps if they're close together, with Lydia you can trap them side by side in her room in Breezehome.) Dismiss her, close the dialogue and tell the other follower to join you (Should say something like Lead the way) then immediately talk to the DB Initiate and do the same (It's gotta say "I command you to follow me" at the top). Not sure if it works for ALL followers, but one of them has to be a DB Initiate.
If you provoke one by attacking it with a weak dagger, the other one will usually attack it. Lydia is pretty weak compared to the DB Initiate (DB in un-upgraded Nightingale, Lydia in 560+ armor. DB Initiate kills me in three hits, Lydia takes a minute or so. Same weapon/one-handed gear). Can use this to test which one of your followers is stronger?
Bump. Once you've done it the first time you can dismiss & recruit the second follower with no problems. (So maybe try Lydia at first since she's pretty easy to trap in her room. Couldn't get Brelnya to work at the start for the life of me)