SIEMENS 840D PROGRAMMING QUESTION

Hey, can someone help me with one small query.
I work with a Milling/Faceting machine which works on Siemens 840D.

I want to know how to program an interpolation on one out of two already synchronized, running axes.

For example: I synchronize rotary B-Axis with the Spindle (its making 50 cuts per rotation[TRAILON]) .

While this is on I want to make an interpolation: G01 Y-5 B50 F250

This does not work. How can i make these sequence of events work?

I would really appreciate your help.

B axis may not be flexible axis, it's may be fixed axis , primary axis can move after locked secondary axis

means b axe locking unlock system, check oem manual