Sure, the whole content of diazepam seems emptied, but the position of both bodies doesn't seem to indicate that was solely the cause - rare that both victims die / lose consciousness at nearly identical times. This does seem to support, rather an external influence - CO2 poisoning or something of that sort, as speculated on by DrAndy above.
Particularly since CO2 poisoning can sneak up on you, the victim being unaware, and for it to strike rather sudden. The scene could support that.