I've been through the difficulties with sidechaining the Q3 in Reaper so I understand it can be more complicated with FF plugins, but for the life of me, I cannot find a way to make the external SC on the Pro MB work for an incoming signal on channels 3/4 on Reaper. I've unchecked the Enable MIDI button, but makes no difference. Is it possible to do? If not, why not? Baffled.
1) Open Pro-MB and at the top-right click "2/4 in 2 out", then click the "+" at the bottom of the pop-up window. This will change it to "4 in 2 out". This has to be enabled before you can assign your sending track to send on tracks 3/4.
2) Pn the channel you want to use to trigger the side chain, click the I/O button and then "Add new send". At the bottom where it says Audio 1/2 --> 1/2, change the 2nd one (after the arrow) to "3/4".
That's all for the Reaper specific stuff.
3) Open Pro-MB again, create a band and expand the "Expert" mode. Change the side chain from "In" to "Ext".
That should do it! :)