How to tag all snowballs that aren't riding another entity?

I want to detect all snowballs that aren't riding a falling sand entity and give them a custom tag called "AbleToStack" set to 1. I also want to detect all falling sand entities that aren't riding a snowball and also give them an "AbleToStack" custom tag too.

Edit: I have found a way, also I actually need to tell you the main idea so we can move on. So, I want do make is so that whenever someone throws a snowball, it turn into a giant stack of alternating sand riding snowball riding sand riding snowball... then a snowball at the very bottom, the initial snowball. Can you guys check my code please?

data merge @e[type=snowball, tag:{AbleToStack:1b}] {Passengers:[{id:"falling_block", BlockState:{Name:"sand"}, tag:{AbleToStack:1b}}]}

data merge @e[type=falling_block, tag:{AbleToStack:1b}, nbt={BlockState:{Name:"sand"}}] {Passengers:[{id:"snowball", tag:{AbleToStack:1b}}]}