I was reading a post I did last October, Intentionally or Creatively Misspelling Names, but I had some recent insight into this topic again and decided to do a sort of update. What I'm about to say goes against my usual beliefs and advice. I do not usually recommend invented names, invented spellings or smushing two names together. However, as I said in last year's post, in the case of smushing for the sake of family, I say "do what you have to do." Just keep in mind, you can put one family name in the first spot and one in the middle.
Mom is Jennifer, mother in law is Vanessa? Jenessa!
Grandma Mary and dear friend Elizabeth? Mariella!
Grandpa Adrian and father Harvey? Hadrian!
However, please note that sometimes you may think you're creating a new name and it is actually a legit old name (Hadrian, for example, is an ancient legit name Adrian came from) and sometimes a smush is not a good idea. For example Jeff and Mark... Jemarco? Not so nice.
My advice is to keep it simple and understandable. Rosetta + Pontia = Rosettia, could be lovely. However, Rosetta + Pontia = Pontetta, lots of confusion.