Five Minds (2021) by Guy Morpuss
The Earth's growing population has finally been controlled. Lifespans are limited to eighty years, except for those who make an extreme choice: to become a commune. Five minds sharing one body, living for four hours at a time. But with a combined lifespan of nearly 150 years.
Alex, Kate, Sierra, Ben and Mike are a commune. They have already spent twenty-five years together, arguing, reconciling, alliances shifting and re-forming. They travel to a Death Park where games are played in which extra lifespan can be gambled like money. The plan is to win time to upgrade their next host body. But then Kate accepts a dangerous offer, and one of them disappears.
Someone is trying to kill off members of the commune. Is one of them responsible? Or is someone else playing a deadly game? It's hard enough to catch a murderer. It's almost impossible when you might be sharing a body with them...