What I expected to happen:
Two double chests facing the same direction to connect.
What actually happened:
They remained next to each other as single chests
Steps to reproduce:
1) Place 2 chests next to each other on top of a hopper
2) Some may not connect into a double chest
Duplicate of MC-127285.
If you hold shift while placing a chest, it will create a single chest, unless you're looking at another single chest (in which case it will connect to that chest and create a large chest).