Is it true that two followers can be used now simultaneously, or is this just a bug involving Serana?

Serana has her own following System. Not a bug. i think



But until a title update. You can use 2 followers as you have Serana

You will be able to use Serana and another follower permanently, other wise there would be a lot of problems for some who dismissed Serana and could not get her back. For Hearthfire you need to be able to change to other followers, so they just decided you could have Serana and someone else.