By default no: It's a block device, so it has no idea what the file system on top is doing. Mounting the same file system in rw mode twice would result in instant corruption unless the file system is built for that. I'm not sure such a file system exists.

What would work is mounting the same file system read only from multiple machines.

You can't run a regular file system like ext4 or xfs but there are file systems designed for sharing a block device: https://en.wikipedia.org/wiki/Clustered_file_system#SHARED-D...

I ask because it would be nice to run a traditional database this way for HA. I suppose nearly as good is being able to instantly mount and unmount across multiple machines if you could also use FoundationDB to fence writes to the device if the old writer comes back as a zombie.

You can definitely do this.

VMwares file system VMFS is designed to do this: https://wikipedia.org/wiki/VMware_VMFS

NBD does not support that. it is a networked block device, the kernel and file systems on top of it do lots of caching. So other clients will not get the latest version if one of them changes a block.