Should likely avoid dividing lengths/areas in perfect halves, as each will likely result in equal frequencies which are additive. Probably better to go 60-40?
That gotcha is if the 60/40 leaves a nice distance with a good frequency itself!
The best is divide by thirds or fifths. That said, half - when the stiffness is really good (plywood) - is, as I said, very likely to be enough. The frequencies of vibration from CNC tend to not be so hard to dampen.
It's also remember that when I'm talking about this, that the desk material is screwed down to the cross pieces and around the periphery.