The NBA Rookie of the Year award has been shared four times in NBA history. The most recent of those came in 2000, when Chicago big man Elton Brand and Houston guard Steve Francis were co-winners, each getting 58 votes.
It hasn’t happened since and likely won’t, now that the ballot goes three deep and is attached to a point system. Also, voters these days are expressly prohibited from casting split votes.
So those are the structural reasons Charlotte’s LaMelo Ball and Minnesota’s Anthony Edwards won’t be co-winners in 2021.
The basketball reasons argue they shouldn’t be, anyway.