The three beds in Dengeir's house are all owned by FalkreathDengeirsHallFaction. As a result, two of the three NPCs who live there (assuming the hold is under Imperial control) can sleep in the double bed at once. I don't think that was probably intentional. The double bed was likely meant to be used by Dengeir only. If that's the case, the ownership on the double bed would need to be changed specifically to Dengeir.