A couple of questions, I'm reading between the lines.
-> is each vice is going to do a different component ?
-> is there any machining done at other than 0° and 180° ?
-> are the same parts to be done on the other pallet in the same position ?
-> if programming around the B-axis, this may limit your available options (ie. part programmed for RH vice cannot be done on the LH vice)
Bringing a program from a vertical to a horizontal then G11 P(angle) would need to be used, this in itself will be awkward for youself to set-up let alone have an operator try to follow it. IMO try using a program that is not transformed
Your answers may open an easier programming option
--a thought, you could use a plane or datum shift (G11 X[±value]) from the B-axis to a local point on the part
PS-- for additional functions for a horizontal-look at this thread (OKUMA MA40 HA variables)