Interesting look at his fancy stats
https://www.expectedbuffalo.com/buffalo-sabres-myth-busting-sam-reinhart-cant-produce-without-jack-eichel/
Quote: It appears that throughout his career Eichel becomes more of a goal scorer when he’s on a line with Reinhart. His goals per 60 minutes at 5 on 5 go from 0.72 to 0.85 and his assists go from 1.28 down to 0.97.
I think the article is very fair, and justifies both keeping Jack and Sam together or splitting them.