How to number collection (non-primary) volumes?

[Hruotland]

Girl Friends: The Complete Collection 1Girl Friends: The Complete Collection 2

How should these two collection volumes be numbered? They each have a number on the cover, but contain other volumes from the original series. (Collection 1: 1, 2, and half of 3; Collection 2: rest of 3, 4 and 5).
On the series page, should they be book 1 and book 2? Or should the book # reflect the original volumes?
(They are marked as non-primary already.)

[Z-squared]

I went ahead and numbered them 1-3 and 3-5, but I also left an explanatory note in the description. I'm open to a better suggestion, though.