They need to "evict" the two HM's one after the other, NOT TOGETHER.
The HM's who already know about the secrets and lies theme, and of course Detective Dan, will very quickly smell a rat if the two HM's "leave the house" together as it's standard on double eviction nights to evict one HM, interview them, and then some time later evict the second HM.
If they leave together some of the HM's might think back to BB5's bedsit with Emma and Michelle.
I just hope the BB producers don't mess this up as it'll be ruined before it's started.