According to
this page, you have a both a DIN-8 serial port (the little round one), and a PowerBook SCSI connector (which I believe is the square looking port with lots of holes and a little nub on top).
So your Duo has two ports: Serial, and PowerBook SCSI
A Titanium, on the other hand, has three types of ports: USB, FireWire, and Ethernet.
First, for your Duo, you're going to need a PowerBook SCSI to regular SCSI adapter. This will simply make your little square port into a regular, long SCSI port. Go to a local Mac reseller store (Not an official Apple store, they won't have old adapters there) and they can probably hook you up with one.
THEN, I suppose, you could hook up an SCSI to Ethernet adapter to your SCSI port... so it would go like this...
Duo<---->PowerBook SCSI to regular SCSI adapter<---->SCSI to Ethernet adapter<---->"Crossover Ethernet" cable<---->Titanium PowerBook
whew! Hope that helps! sorry my post was so long-winded.