I've got one MS 900 and a y-connector. Will definitely get second MS someday soon and may get the third (helmet mount) but probably not. The single MS is so awesome that I simply can't imagine needing more than two. But that's what I said about women. YMMV.
Edit: The MS lights are strictly 7.4V, if your waterbottle batt is 6V or 12V it will not work.