Multiple Employer Plan, Employers Split Up - Should Transfer Occur ? Options

[JAY21]

I'm trying to think through a situation of 2 plan sponsors who sponsor the same plan (initially was an Affiliated Service group I believe). One of the employers (small medical P.C.) who co-sponsors the PS plan is now splitting away from the main employer and plan sponsor.

Since his P.C. was a co-sponsor of the plan do we have to apply vesting against his distriution/rollover to a new plan for his P.C. ? Or since he hasn't terminated employment from his practice (same M.D., P.C. is still ongoing) should it be treated as a spin-off/transfer equal to the total contributions allocated to his practice share of total assets ? Maybe like a 5310 transfer though I think DC to DC plan transfers don't require a 5310.

Thoughts anyone ?