With employees (assuming a separate entity is setup), make sure if there are any who work at both locations (will likely come up sometime, so prepare in advance), that they need to be "hired" by the other location, and they need to be treated completely separately. Do not get caught using one paycheck for both places, not having separate paperwork (I9, new hire report, etc) at each location, etc. Since they will want to have the two entities completely separate, they really need to be, including payroll processes.
Things to watch for/talk over: Garnishments (make sure to only apply it to the business which received it), OT (two businesses, employee could work 8 hours a day at each, with no OT), WC rates (if low at the existing shop, it will likely be more at the new shop), how the owner gets compensated from each, and so on.
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I was going to spout on certain upcoming CA ammunition laws, but it gets me too riled up, so I try not to think about it...