FYI, on latest version I can not add a second disc... Enter the Matrix NTSC (Disc 2)
http://redump.org/disc/6773/
$ wwt -vv add --part /dev/sdc1 GMXE70-disc2.iso
wwt: Wiimms WBFS Tool v2.00b r2855 x86_64 - Dirk Clemens - 2011-07-13
One WBFS partition found.
1 object scanned, 1 disc...