I used to do the same with the Soundpool M04 and a printer, but I dis-connected the printer long ago. There should be no reason why it does not work.
I am actually thinking about sharing the parallel port again too, so obvious things to deduce the problem:
1. That both ports work on your parallel switcher work. i.e. try both printer and SMP24 on both ports
2. You have definitely enabled the SMP24 MROS driver in the MROS folder? i.e. .DR extension has been renamed .DRV.
Try doing it again just in case.
3. SMP24 on its own has no problem with connection without the printer and switcher?
4. Tried alternative or centronics parallel cables?
I am sure you have checked the above!
Atari Falcon 14mb, 68882, Dual 8gb CF, Steinberg FDI & Analog 8
Atari Jaguar, Rotary controller, Skunkboard & Cat Box
Atari 520STFM 4mb, TOS 2.06 switcher, OverScan, GigaFile, PARCP-USB, Unicorn-USB, System Solutions MiniS HD, SyQuest drives, ICD Link II, PhatBoy MIDI Controller, Philip Rees 5M MIDI merge box, SoundPool MO4, Steinberg MIDEX, SMP II, Emagic Log 3, C-Lab Unitor 2, Combiner & Export expanders