Before Reciept date: A
Between Receipt date and Approval date: A and/or B. (1) (2)
After Approval date: B, unless you checked "concurrent employment" on I129.
(1) You is B's employee from the the actual date you started working for B.
(2) My understanding is you need to be paid fully by A or/and B, otherwise they violates the terms on the petition (prevailing wage).