Is it possible to merge two demons into one as spirit
Yes. That’s what the so-called “meta-gods” that people create are. The “Goetikon” for example, is all 72 spirits of the Goetia merged into one.
I was wanting to create astaroth Lucifer how do I start that process
Yes but they don’t literally merge, they just work as one to aid you or you go the route where you create an egregore that’s based off both.