For example, a specific wiktionary versus a specific wikivoyage project
Topic on Talk:Wikistats 2.0 Design Project/RequestforFeedback/Round1/Component wiki selector
Yes, especially in the same language.
Interesting, can you give an example of how you would compare that and what you would look for?
For example, I may want to look which sister project is more active for a given language: e.g. I can find out that French Wiktionary has more active editors than French Wikivoyage, while Finnish Wikivoyage has more active editors than Finnish Wiktionary. Typically I can look at metrics that have similar interpretation (pageviews, active editors) but not at those that have different interpretations (number of articles, for example). This is not my primary need however, and it is OK if this data will be available only in tabular format (assuming tables are compatible for different projects).