Qemu as an NBD server advertises MULTI_CONN support (that is, clients may open up parallel sockets, which will all see cache-consistent data over those parallel connections); but does not take advantage of parallel sockets as a client.  Rich Jones attempted some RFC patches a while ago:
https://lists.gnu.org/archive/html/qemu-devel/2023-03/msg03320.html
but these need to be modernized and take advantage of ongoing multiqueue work (parallel sockets competing for the kernel via coroutines is not going to scale as well as actual parallel threads).