Okay, to answer my own question a bit and hopefully help out anyone
else who has the same issues:
First it was necessary to enable multiple hand tracking in the OpenNI/
NITE drivers generally. For me, this involved changing the following
files:
/usr/etc/primesense/Hands_1_3_0/Nite.ini
/usr/etc/primesense/Hands_1_3_1/Nite.ini
/usr/etc/primesense/Hands_1_4_1/Nite.ini
/usr/etc/primesense/Hands_1_4_2/Nite.ini
/usr/etc/primesense/Hands_1_5_2/Nite.ini
It might only be necessary to change one of the files (the most
recent?), but I changed them all just to be sure.
I had to remove the semicolon comment ';' from the start of the lines
in these files so that the lines just read:
AllowMultipleHands=1
TrackAdditionalHands=1
After doing this, OSCeleton could 'find' more than one hand at a time.
However, it still only sends OSC messages about one hand. A look at
the source code suggests that the program just doesn't have support
for tracking more than one hand written into it (i.e. there is only a
variable for tracking one hand, and the "l_hand" message is always
used). I'm going to play with the code to see if I can get messages
for r_hand to work, and I'll report back...