Not sure what you mean by greyed out? If you want split cue make sure the knob/fader is in the middle where ch1, ch2 are

That's not split cue. Split cue is having the master on one side and the program cues on the other. It's similar to having one headphone on and one off. Some people use it instead of a monitor or with in-ears. I would have preferred a quick switch between master and cue thou.

Its grayed out in serato control panel.

My bad I thought split cue in the software was only for controllers.

Maybe it's only for use with controllers? Kinda like how SDJ was only compatible with controllers and not DVS till later. It was originally a controller based software. I'm curious now cause It would be cool to use split cue but not sure how this would work with the S9.

Anyone know if this can be added with a firmware update?

I believe it's a hardware feature, might be wrong, but it would be pretty hard to do if it wasn't already built in.

There is no split cue and there won't be a split cue on this version of the S9. I don't miss much from moving from the 62, but I do miss split cue.

It's greyed out because pioneer has other hardware that utilizes split cue. From what I've seen, all of the new mixers they have released lately have come with split cue except for the S9 and the controllers. It's a little weird.