Can I assume that it is 2 tests of 45 minutes with a break in between?
That the score is made up, as the Bham one is, of 50% marks for literacy/VR, 25% NVR and 25% Maths?
Or is it different?
Yes, it is 2 tests of 45 mins but no, it was different from Bham in that Literacy, NVR and Maths were all equal thirds of the total mark last year (although anything is subject to change in any year I guess).
I have had 2 children take the Warks CEM exam: one last year and one 2 years before that. My impression is that the advice on the Bham section generally also applies for Warks, so worth having a read. Plus last year there were several reports posted by parents of children who took both Warks and Bham that the exams were very similar in style. Not the same questions though!
I think some preparation does help but it is difficult to tell. Keeping it varied and throwing new stuff at them is probably a good idea. I expect you are aware that in other grammar school areas the test is formulaic and children gain huge advantage from being tutored to answer questions of a rigid style, but the Warks test is not so predictable. There is usually huge time pressure, but last year was rumoured a bit less so. "Keep Calm And Carry On" is also good advice. My second son only managed to answer half the questions in several sections last year but still got his grammar school place. As Ed's Mum said, for children who are used to coasting at school and getting almost everything right, it can come as a shock to be confronted with more difficult stuff and it is important not to panic. If your child is a genius like no_ball's then you have no worries as they will pass whatever you do or say. If they are mortal like mine, then a bit of forewarning about not giving up might be a good idea!