I read theres a glotch where you can have serana as a full companion (carries stuff) plus another full companion? Was this fixed in 1.9?

I tries it in the castle before she opened soul cairn but age said was to impotant to share?

I don't know of any glitch and I think the only way you can have her as a follower PLUS another follower is when you first find her. After that she won't join you until you dismiss your current follower. Only PC follower mods can overcome this issue allowing you to take her and another follower at the same time and be able to give both of them things to carry. Again, to my knowledge. I'm on the PC so if there is a glitch for consoles, I won't know.