Diablo II patch 1.12a can now run without CD's in the CD-ROM drive, however, if you installed Diablo II prior to 1.12a, you don't have this option.
There's a simple fix though!
Simply copy all the files off your D2 CD's, into your D2 folder, and replace the D2gfx.dll with the one provided below.
Enjoy multiple Diablo II's! (Great for Open B.Net character editing)
D2gfx.zip