So the spine of the arrows should match to the Bow weight calculated after draw length?
Yes
I do like my arrows long. I do not have to add to spine for these bows. No FF strings.
How long?
You'll be hard pressed to find one arrow to work with both bows. If you get a set of arrows to work in the 50lb bow with 125gr heads you might get by with 145gr heads for the lighter bow. You could get a set to tune in the lighter bow and use the same arrows/heads in the heavier bow but build out the strikeplate to tune.
George's suggestion above, to me, is correct.