As YVR Cockroach has said, as long as you've booked it as a double (ie: "2 persons in Deluxe Double cabin" on the bahn.de booking engine) then it's all yours.

The deluxe compartments on that train can be configured as 3-berths, but in your case, the 3rd berth will be folded out of the way, and there'll just be two berths made up.

However, if you'd booked it as, say, "2 persons in Deluxe 3-berth cabin", then it would be possible that a third person (of the same gender) would be allocated to the same compartment. However, I don't think bahn.de will actually offer "2 persons in Deluxe 3-berth cabin" as a booking option (even though it is theoretically possible) - and if you had booked that, it would have asked you to select your gender (while if you book a complete compartment, it doesn't).

If you booked on a site other than bahn.de, then I am not sure which terminology is used, nor quite what the booking process would offer / ask.

Enjoy your trip!