Try to do it in some more steps.
First, use the converter filename - tag and put the complete first number into the tag TRACK, so that it contains e.g. 101
The create 2 actions of the type "format tag field", one to format DISCNUMBER with the formatstring:
$div(%track%,100)
This gives you the result of the division as an integer, in this case "1".
Then the same type for TRACK:
$mod(%track%,100)
This gives you the remainder of the division of 101 by 100.
These two actions in one action group work for all joint disc/tracknumbers up to 999, that is 9 discs with 99 tracks each.
Note: Replace each special ÷ character with one space character.
End Action Group Test_2011#20110727.TagFieldImport (1 Action)
From:
_FILENAME = 101 - Sonata No. 1 in F minor, Op. 6, I. Allegro con fuocoso.mp3
To:
DISCNUMBER = 1
TRACK = 01
TITLE = Sonata No. 1 in F minor, Op. 6, I. Allegro con fuocoso