Is there a benefit in developing and ascending a character, then using that upgraded hero to level up another ?
that is not worth the feeder heroes and ressources spent on the one that you wish to feed to the other
You actually lose 90% of the XP of a hero that is used to level another hero.
I often recommend spending gems to buy 15 roster slots ( 5 slot purchase x3 ) so you can keep extra 3* heroes to level up when your current hero team is max. Always keep at least one of each 4* & 5* heroes is good too. As Rook says, the Devs are always rebalancing under used and over used heroes.