So how the heck do landed titles work. Reading wikis like mad and I still don't get it.
I started as a lowly dude (count? I think). Had three sons. My liege lord decides I've been pretty great and grants me another county. Huzzah! Except wait, now I have a notification that upon succession I'll lose this new county. Well that makes no sense! I check on it, and it looks like my eldest son (my heir) is scheduled to inherit my original county, and my middle son is scheduled to inherit my new county. I wasn't allowed to give the title to the new county to my oldest son, but I could give it to my youngest (third) son.
You can clearly hold two titles at once, since I myself was holding them. Why couldn't my heir inherit both?
(fun fact: eldest son died right before I started an ill-advised war, second son was taken prisoner and sacrificed to the Drowned God, third son was sent to the nights watch. So my heir is now my grandson, only child of my first son, who is 12. My only other living descendant is the legitimized bastard only child of my third son. Things...things do not look good for the First Time Playing family.