Steve Perry wrote:
Yup, as amfoto1 says, there are companies who have done it - I had the one from Singh-Ray in fact. The trick is they are more expensive and if you don't really need one or the other, than it's more cost effective to skip the combo.
The bigger issue is however, that the stacked filters are thicker and often vignette on wider lenses or the wider end of zoom lenses (I couldn't use the one I had on my 24-70 at 24mm~28mm).
You could, however, have the filters in a bigger diameter and use a step-up adapter - also not a bad way to economize a bit with filters if cost is an issue