I was messing around with command blocks then I thought what if I was able to summon a falling sand entity on another falling sand entity so I tried it and it only summoned the top one. Here is the command:

/summon FallingSand ~0 ~2 ~0 {Time:1,TileID:137,Riding:{id:FallingSand {Time:1,TileID:152}}}

Then I tried summoning a falling sand entity riding a cow and it worked. Then I thought what if it is a problem with summoning something riding sand so I tried summoning a cow riding falling sand. The cow fell and no sand appeared. Here is the command:

/summon Cow ~0 ~2 ~0 {Riding:{id:FallingSand {Time:1,TileID:137}}}

I don't know if this is for any other versions but it is on 1.8

I'm on a Windows 8.1 laptop. And I have a quad core 64 bit processor.