In vanilla Minecraft, players cannot open the double chest at all if one or both halves are blocked. But plugins should probably be able to access the full double chest anyways regardless of whether or not the chest is blocked.

I haven't checked the behavior of older server versions yet.

Dummy test plugin code can be found in https://hub.spigotmc.org/jira/browse/SPIGOT-6759
But this is a separate issue to SPIGOT-6759 that is not fixed by the PR attached to SPIGOT-6759.