And also just be careful that they don't get in each other's way in combat. Just be careful that they don't literally get in your way as you try to navigate smaller spaces. Get a hundred conjuration to get ability to summon 2 Atronach / reanimate.This guys light a torch whenever it's dark!) Laid to Rest (speak to Thommir in front of the cave.The Sanguine Rose summons another Atronach.Any ability that summons a temporary helper.A horse like Shadowmere from The Cure For Madness.The Blessing Of Nature (There is a guy who asks to join you when you initiate the quest.).
Quests in which you get an NPC who doesn't replace your existing follower that can stick around include:
(Occasionally, Delphine will try to leave, but simply tell her to follow you.) Combine these NPCs with a Conjuration or two (must be at max level), a horse, and a follower, then you've got a full traveling troupe of insanity. In Alduin's Wall, two seasoned fighters tag along. In A Daedra's Best Friend you get to recruit a rather reliable dog companion named Barbas. They will remain in your party forever until you complete the quest or dismiss them. You can trick the system by going on certain quests to recruit people into your party but not finishing the quest.
