doable but, near unimplementable do to the lack of
Qbus. The more common
situation is unsupported, IE: DEC didn't consider it marketable or test
it exactly that way. Two RDxx disks in a ba123 is an example.
Two RD5x's in a BA123 was definitely a supported configuration, and is
listed in the late-80's DEC catalogs.
Two RD3x (half-height) disks in a BA23 is also supported, and indeed is
the reason behind the 6-button BA23 front panel. Many 11/53
configurations were shipped from the factory this way.
Two full-height hard drives in a single BA23 was never officially supported
(at least for the various microPDP-11 configurations), because
they refused to support a base box sold without removable media.
Tim. (shoppa(a)trailing-edge.com)