A bit late to this...
It gets interesting in regional UK races. Round our way in the last couple of years there has been problems getting a big enough field for a women's race, but holding it on the same day as a men's race gets around some of the cost issues. Running them simultaneously has the advantage that the accredited marshals etc. only have to be around for half of the day, you are only disrupting the area you are racing in for the morning, and you don't need the hall for so long.
However, you do need twice as many cars, and additional other officials (commissaires, NEG, lead car drivers) so you aren't halving the costs.
One I did last year was supposed to have a 10 minute gap between races, but looking at the field the decision was taken to roll out together, then de-neutralise the men seconds before the women. That meant the women couldn't do a breakaway from the start, which is unlikely anyway, but they were unlikely to be lapped. It got close to the second happening