I checked twice, that if i put a double-chest and put another two chests on top of the double chest - they are separate.
MC-154355
Can't place double-chest on top of double-chest
Resolved
Duplicate
0
Duplicate of MC-127285.
When sneaking (holding shift by default), a single chest will be placed, unless you look at a single chest (in which case it will try to create a double chest together with that chest).