Step 1: Slice your Moleskine in half!
Originally I was going to cut my Moleskine (actually three of them--a standard pocket sketchbook and two of those skinny cahiers) in half myself using an X-Acto knife a few pages at a time, but as I was driving home from the craft store and thinking about how much of a pain that was going to be, it occurred to me what short work one of those big automatic paper cutters would make of it. So I went to Kinko's and asked if they had a "really big paper cutter," to which the woman behind the counter replied, "Yeah," in a voice that really meant "Duh!" (though not unkindly).
She charged me $1.50 for each cut, so $3.00 plus tax for the three notebooks (she stacked the cahiers and cut them both in one slice), which seemed well worth it for the trouble it would have taken me.
A couple notes about letting other people cut your Moleskine:
1) Measure it yourself. I asked her to cut them each in half, and I saw her working with a ruler in the back, but one half came out an eighth of an inch wider on all of them. So I recommend doing the measuring and math yourself, and then clearly marking where you want the cut made before you hand it off for chopping.
2) Be specific about the elastic band. I asked her to just cut the elastic band in the center because I wanted each half still attached, so I could stretch each half around the back (I think it was long enough) and have an elastic band for each Mini Moleskine. She misinterpreted that as cutting the elastic band off completely, which she deftly did before I could stop her! You can see the result in this photo. Not a disaster, but a lesson learned.
3) Don't forget about the bookmark. The Moleskine was brand new and I completely forgot that the bookmark was neatly tucked inside. As a result, I now have a slightly-too-short bookmark attached to one Mini Moleskine, and an unattached bookmark for the other. Oops.
The cut wasn't perfect. On the new edge of the back of the bottom half the black oilcloth got torn a little, which you can see in the center right of the photo. But it's a lot better than I could have done with my X-Acto knife.
Of course, now I have two Mini Moleskines with half-pockets in the back. I'm not sure if I'll leave these as-is--they still hold slips of paper reasonably well--or try to fix them somehow. It oughtn't be too difficult to close up the now-open sides with an accordion-folded piece of paper that would look and work almost as well as the original.
The cahiers originally had weird half-pockets, so now one half has a half-pocket, and the other just has a cardboard flap (see photos). This might be worth fixing, but I haven't gotten around to it. A staple would do it, but stitches would be more attractive.