To get the combinings of the Green Guides right is a daunting task (I haven't started looking at other series yet!). If you are planning to combine a volume, please remember that Michelin have changed the Guides significantly in the past 20 years. Each volume used to cover a smaller area than some of the current set (e.g. while Brittany covers the same area, Burgundy/Jura used to be represented by two volumes - Burgundy/Morvan and Burgundy/Jura). These major changes apart, succeeding volumes were revisions rather than re-writings so, I think, can reasonably be combined but please take care that you are combining like with like.
Red Guides should not be combined unless they are from the same year, so that Series can be properly made.