I just watched BBUK 2017 and I think this is kind of what inspired the "attic twist" for BBAU 2022.
Here's how they handled it: five were up for eviction, the housemates thought it was just a normal eviction, it ended up being a double eviction where the two people with the most votes left (it was vote to evict) - and they did both leave.
The SAME night, the houesemates were told they were going to evict another two HMs (it was finale week so people didn't suspect it was fake) with face to face nominations. Each person had to choose a card secretly with the photo of their desired HM to evict, and the two evicted were "fake evicted" and sent to the attic, where they could spy on the housemates and had full control in the final week's theme where the HMs thought the public was choosing their finalists, and they had control over tasks (make fake questions for them in the diary room, pretend to be the audience vote, etc) - it was engaging, fun, and no actual evictions were reversed (so it didn't feel like producer manipulation)
BBAU, learn from this!