So, I got to thinking about the merging and how the two don't really seem to match in the two movies. How it seems like Dark Alessa was merging with Sharon/Heather twice. But hear me out; what if at the end of the first film one part was still out. I mean, in the movie there were three parts; the original form (Alessa), the evil (Dark Alessa), and the good (Sharon). We were only hinted about the one merging in the first movie. So why is it not possible that at the end of the first film only two parts merged and in the second the third part finally completes it? For all we know Dark and Sharon merged giving Sharon the reason she can snap at someone in the second movie because there is anger. But Alessa is still around and she is still full of hate because the cult is still around. And because it is the original part, she just grows in power giving her all that strength.
That could also be the reason why the two have such different looks as well, one was dark, the other was actually Alessa using her powers to take on a different form.