How do I tag this specific entity? (Go to description for info)

I want to merge the snowball/FallingSand passenger nbt tag with FallingSand/snowball.
So, I have two commands set up:

data merge @e[type=snowball] {Passengers:falling_block, tag:{BlockState:{Name:"minecraft:sand"}}}

data merge @e[type=falling_block, tag:{BlockState:{Name:"minecraft:sand"}}] {Passengers:snowball}

The commands are supposed to detect whenever a snowball exists then make it so that a FallingSand is placed as a passenger. Then, it places snowballs on all FallingSand, and this repeats, forming a snowball/FallingSand tower. I’m not sure if it will work, since it probably will detect the non-top snowball/FallingSand and break the chain by removing all passengers then placing a FallingSand/snowball on top. Is there a way to ensure this doesn’t happen?