There is a small window of opportunity to acquire two followers at the beginning of the quest 'Chasing Echos' just before entering Soul Cairn, Serana and one other. From the point where Serana is waiting for you to fill the vessel with the special ingredients, until you decide whether you want her to turn you into a Vampire Lord or capture your soul, you can leave via the balcony door. You now have the opportunity to leave the castle grounds by fast traveling somewhere and recruit another follower. Once you do and fast travel back to the castle balcony, Serana will automatically rejoin with you and your follower will not be dismissed but forced to wait on the balcony. Your follower will not enter the Castle (neither will a dog companion). However, when you finish with the Soul Cairn and leave via the balcony again, your recruit will still be there waiting. Serana will continue traveling with you. If you take advantage of this one opportunity before completing "Stage 100" of this quest, you can have two followers for the rest of the game. You will still be able to change the 'normal' follower, dismiss him or her and pick another instead, continuing to have two followers in the group, but you can't dismiss Serana. If you dismiss her this opportunity will be lost.

Also if you dismiss your second follower before you enter Ancestor Glade when starting with quest 'Unseen Visions' you will lose the second slot for that quest, therefore you need to tell your second follower to 'Wait Here'. However there will be a second opportunity at the end of 'Touching the Sky' immediately after you kill the boss for that quest to fast travel elsewhere and return with a second follower - at which point you'll have a second slot for the rest of the game, although it's recommended not to dismiss them until you've killed Harkon at the end of Dawnguard and returned to normal game areas. Dismissing Serana at any time will always eliminate your second spot so don't do it, ever.

See the notes on https://en.uesp.net/wiki/Skyrim:Chasing_Echoes

and https://en.uesp.net/wiki/Skyrim:Unseen_Visions what to do to keep your second follower

and finally to get the option for a second follower back read the notes on

https://en.uesp.net/wiki/Skyrim:Touching_the_Sky