Providing an update on this. While mounting a volume to more than one droplet sounds great in theory, this does not work well with the design of volumes / block storage. The design is that the droplet will see the storage as a locally attached drive, exactly the same as a physical hard drive placed into the computer. For the same reasons you cannot place a drive in one computer and also attach it to the computer next to it, block storage cannot be treated that way either. The operating systems are simply not built to function that way, the data would be corrupted.
This isn’t to say that a future product iteration would not function differently or that a method of working around this will not exist later. However, it is to say that right now this is not the best way to approach the concept.
Now there may be a way to do something like this with object storage. There are definitely ways to add another layer on top of the droplet which has the volume attached, that can be used to mount it elsewhere. SSHFS was one thing mentioned here, NFS being another.
Hope that helps provide some clarity on this :)