When trying to summon a falling block with a passenger of another one falling block, the top one gets offseted for some reason. And if you manage to find the rigt spot the second falling block places like normaly should.

Here is the demonstration: https://streamable.com/0g52q

Here is the command to try it out for yourself:

/summon falling_block ~ ~2 ~ {BlockState:{Name:"minecraft:stone"},Time:1,Passengers:[{id:"minecraft:falling_block",BlockState:{Name:"minecraft:activator_rail"},Time:1}]}