Is there such a thing? I don't care about performance, I just need to be able to have access to the same block device from two computers. The block device does not have anything fancy (like reservations), so it's pure read/write device without caching or other nasty tricks.
They are named cluster filesystem https://en.m.wikipedia.org/wiki/Clustered_file_system . There are different products with different features. For instance: GFS2, OCFS2, Lustre
Caching is a layer above an actual problem, conflicting writes to the disk. Say node A and node B are presented the same disk. They try to write different things to the same blocks at the same time. Very likely this corrupts the file system.
Instead, use a network file system like NFS or SMB, which support multiple node access. Many full featured storage systems can present volumes as file shares.
Cluster file systems such as GFS, GPFS, or OCFS, also exist to address concurrency problems. Their designs tend to require nodes have a distributed lock manager and fencing mechanisms (those fancy SCSI reservations) to make multiple node block access safe. Historically, cluster file systems have the reputation of being more difficult to implement, compared to a file share with logical file access going through one server.