Deathwatch do work in teams, usually led by an Inquisitor but occasionally by a Librarian or Captain, and as representatives of both the Inquisition and one of the most sacred of Astartes responsibilities they are allowed to requisition anything they need to get the job done.

I haven't yet seen anywhere that says how big a 'Team' is allowed to be, and some books I've seen have sometimes referred to them as being reasonably large forces. Similarly, there's nothing I've seen that says an acting Deathwatch Commander can't request MORE Deathwatch members being recruited or recalled to service in extremely dire circumstances (or just requisition the nearest Space Marine company for reinforcements).

It's thin, but it's plausible. It'd certainly explain the use of uncommon and xenos weapons in the force and of course - depending on which codex you want to use - it'd be quite possible to create a very small, elite army; Draigowing or something. 30 models in a 2000pt army could easily be interpreted as a couple of Kill Teams working together, for example.