Not sure how best to describe this situation, but I'll tell you what I'm doing and how I've solved it in Ableton Live; I'm hoping there's an equally simple solution in Reaper that I've just missed. Hope it makes sense!
I've got a Timefrog 2 sequencer with USB MIDI out. I have a Meeblip hardware synth which just has a 5-pin MIDI connector, and I have an Edirol UM-2 MIDI interface which connects to the Mac with USB, and has two 5-pin MIDI outs. Both the Timefrog and the UM-2 are connected to the Mac with USB, and the UM-2 is connected to the Meeblip with a 5-pin MIDI cable. I want the Timefrog to control the Meeblip via Reaper so I can record both the MIDI being sent by the Timefrog and the audio coming in from the Meeblip. Sounds simple, but I can't figure out how to set it up in Reaper. (By the way, I know there is another way I could cable this up and get it to work straight away, but since I can get it to work in Live with this particular cable setup, I'd like to do it the same way in Reaper).
In Live, it's a matter of setting up a MIDI track with input from the Timefrog, dropping an External Instrument module onto the track and setting the "MIDI To" option to the UM-2, then creating an audio channel with an input from the relevant vhannel on my Duet. This means the MIDI channel in Live is accepting the Timefrog's output, and pushing it back out of Live via the external instrument to the UM-2 and therefore the Meeblip. What's the best way to do this in Reaper? I've tried doing it through the MIDI Hardware Output section in the relevant track (ie, set the track up to have MIDI input from the Timefrog, then set the Output up to go to the UM-2, but it doesn't work. I can see that MIDI is being received by the channel but I can't get it back out of Reaper and to the UM-2 to control the Meeblip. I've hunted through the JS plugins to see if there's something there that acts the same as the External Instrument in Live, but I can't find one.
Any help would be much appreciated! It's more out of interest at this point than something I'm desperate for, as I've done what I needed to do in Live, but for future reference it would be handy to know where I'm going wrong. Cheers