OK, as this comes up a lot I've DLd Reaper and had a play and this is how I got it to work for me on the Mac.
In addition to Reaper you need some kind of virtual midi port. I happened to use MidiPipe (so I could see what was going on) but the IAC would probably work just as well. On Wndows I'm out of touch but MidiOx/MidiYoke always used to work for me back in the day.
1 - Set up you virtual midi port for one input and one output.
2 - Then launch Reaper
3 - Create a new virtual instrument track to host the Noatikl plugin. Leave all the Reaper settings for that track at default.
4 - When Noatikl is open, set the Midi output = your virtual midi input (Midipipe 1 in my case). Midi input should be the sequencer midi ipieline and check the Sync? and Play this file tick boxes.
5 - Create a track for your VSTi(s) and load your synths of choice.
6 - In the input list (under the track volume slider) select -> Input Midi -> Your virtual midi interface ->Channel 1
7 - Create the track for your next VSTia nd load the synth of choice
6 - In the input list (under the track volume slider) select -> Input Midi -> Your virtual midi interface ->Channel 2
7 - Repeat up to channel 16 if you like.
8 - Set up your voices in Noatikl, being sure to allocate the correct midi channel to the synth you want to address.
9 - Hit play and admire your handiwork.
Below is a screenshot of how it looks here running two instances of Crystal on channels 1 and 2
Hope that helps!
[edit!! - Forgot to say that you need to go to the Audio and Midi setup in Reaper once its launched to enable the virtual midi port for input and output, otherwise it won't appear in the drop down list on Reaper's track page]