Store twice as many CDs in the same space.

Step 4 Tranfer the U-card and cover note.

Now take the U-card from CD case 1 and lay it on the one in CD case 2. Does it fit? If not, put it back and try the same with the U-card from CD case 2 in CD case 1.
This is the part where you'll have to determine which U-card goes in first.
The flat part of the U-card, between the folds, varies in width from case to case. The narrowest one should go inside. Sometimes, they're the same width. In that case, you choose.
A wider U-card won't lay flat in a narrower one, which can cause problems later.
Once you have determined which card is the narrowest, lay it upside-down on the table with one folded end off the table (image 2, unless you know a better way), then fold the end 180 degrees (image 3).
Repeat for the other end. The card should now be "inside out".
Lay the U-card on top of the other one in CD case 2.
Now it's time to transfer the cover note.
Pull it out, flip it over (image 4), then slide it in on top of the other cover note (image 5).
Add the now-empty CD case to your Stack-O-Empty CD Cases (image 6). You may need some of those later to replace damaged cases, especially when dealing with damaged double CD cases you may already own.