We Have this option on our Multus B400W
what option do You have?
M110 L spindle C axis mode ON
why You need C axis?
are You sure, that M889 and M888 are usable here? I'm asking, because I don't know. I imagine this operation quite different way.
You need to have possibility to synchronize both already synchronized to each other spindles to Z feed to cut thread that way. Are You sure, that You have that option on Your machine?