I have a MIDI splitter/din sync box that I bought on eBay. It has one MIDI in, 6 MIDI outs, and 2 din sync outs. I believe the correct term, which I should have originally used, is a MIDI THRU box. A THRU box has 2 or more MIDI outs and should carry all data received at its MIDI IN on any of the 16 possible channels unless its manual says otherwise. I have never used my THRU box to send data from my D-550 to any units because it is a rackmount unit with no keys. The only thing the D-550 needs to output MIDI to the PG-1000, because a two-way communication is required for the PG-1000 to work.
The setup I described in my last post will let you use a computer or another keyboard and a PG-1000 to control your D-50 as a sound module. If you want to be able to send note data from the D-50's keyboard to the computer, plug the MIDI out from the D-50 into a THRU box. Plug one output from the THRU box to the "parameter in" on the PG-1000 and another output from the THRU box to the MIDI IN on your computer interface.
Before you buy a MIDI THRU box, check to make sure the MIDI THRU on the PG-1000 doesn't mirror the data received at its "parameter in" instead of its MIDI IN. I really don't think it does, and thus you should have to have a THRU box to send note messages from your D-50's keyboard to the computer, but it is not necessary to have a THRU box to send note messages from you computer to the D-50.
Hit me back if you have any more questions.
-Lee