you need to issue the sync spindle G/M code.
it can depend on your specific control.
M303/M304 - MAIN/SUB spindle active for spindle control commands
M34 Dual synchronize spindles turning mode – Phase control C-axis chucks
M35 Dual synchronize spindles turning mode – RPM speed only
M36 Dual synchronize spindles turning mode – CANCEL
will do it.
or something else. check your manual because Mori Seiki special G/M codes are usually different than Fanuc codes.
The other option is just unclamp 2nd chuck then rotate the part while it is being held by 1st chuck then clamp 2nd chuck.
M69 (sub spindle unclamp or whatever your code is)
M68 (sub spindle clamp or whatever your code is)