It is very possible for the UN-supported modified versions of OS9 to see the modem in an iMac but not in an iBook. They are not the same hardware. Even getting the sound driver to work properly in the iBook is a known problem.
If the modem module in the iBook is damaged or not connected properly, it's possible the OS9 installer didn't "see" a modem during the installation and so never installed the modem drivers / extensions. You should check if those Extensions are installed and compare it all with the iMac. Also, remember that the hardware for a printer and/or modem was very different in older Macs –that had built-in serial ports for them – which your iBook doesn't.
* This is ALL guesswork. I am not a former Apple employee with detailed knowledge of the hardware and firmware designs of old Macs and even if I was, I would probably be constrained by a non-disclosure agreement anyway. So…
My best suggestion is: If you can't find an obvious reason for all this like what I've suggested above, you be satisfied that at least you have one working serial port to use. Do you really need two Timepieces running on a 12" laptop? Personally, that sounds pretty crowded to me - driving that large of a network and many tracks on a tiny screen although… You CAN get 32 individual MIDI ports by stacking the Timepieces on the single Keyspan port that does work. You are unlikely to actually get into trouble unless / until you try to run a LOT of busy tracks at the same time.
Anyway, I'm a little puzzled by what information you've provided so far. You have an iMac with two Studio 4's and now you ALSO want two MTP's on an iBook? Do you own two sets of 20-odd MIDI devices? Since that seems unlikely, what are you actually doing to need two large setups? Possibly the iBook simply isn't the best choice to begin with.
