On a lark, I tried getting one of the Companion quests from Farkas where he becomes your follower - kill a dragon... I was successful in having him give me the quest, and I was expecting to have Serana dismissed - but no.... I wound up leaving both for the quest and when it was over, Serana was still with me.
So.... I went. Back to Jorrvaskr and asked Aela, my old faithful, and lo and behold.... I now have two followers. We'll now see if they can peacefully coexist. (Serana killed Bran after some battle where he may have nipped her or whatever....) I expect trouble....