Seems to me it should work, the SECURE Act language just says adopt “a plan”, not “a new plan”. But AFAIK there is no guidance. Another option would be for B to simply adopt its own PS plan and then permissively aggregate the plans for testing.
I've figured out that because it's not actually a sale but a closure because the owner is retiring we have to maintain for the full year. They will actually be operating at a loss this year so there is always that way out.