Do the regs allow two DBs to be connected in parallel protected by only one
i.e. a cable feeds DB1 then another cable goes from DB1 to DB2 but connected
directly to the feed for DB1.
I've looked in the regs but cant seem to find anything that prohibits it.
Nothing I can think of explicitly prohibits it.
There is a requirement that faults be limited as far as possible in
their effect on other circuits, and clearly in this case a fault in
one DB will effect the other as they share the same circuit protective
device. You should not downsize the conductor size from DB1 to DB2
(unless you can show there's no chance of overload and it can still
clear a fault current within the required time).
Also note that fault discrimination with upstream MCB is not very easy
(even when they aren't shared), so a fault current in one of the DB
final circuits might take out the upstream MCB too, which in turn
will take out both DB's. You would need to examine manufacturers data
sheets carefully to ensure discrimination, and you might find you have
to use a BS1361 or BS88 fuse to provide correct discrimination rather
than an MCB, depending on ratings and PSCC.