JohnKenn
My problem with the obvious chaining behavior in Reaper is that the merge input midi bus function creates a wrapper for a pseudo parallel routing dealing only with initial input issues. It's function to take the input and route around a synth that has no midi thru capability and would thus block midi notes to the next synth in the series chain. Thus both synths are receiving midi input but are still ranked in a series chain. Works great for getting all synths in a chain to sound.
A disadvantage to total freedom in routing would be for instance if you had synth A and synth B with no midi thru, and adjusted by merging the midi input as if both synths were first in the chain. Then you wanted to apply a reverb to synth B only. Can't do it because the synths are still in a series chain even if receiving parallel input. Reverb will process everything upstream equally. Inability to apply panning to an individual synth is another problem.
OK, I think I just did what you are talking about using only REAPER's native routing.
I set a track up for midi recording and set it's number of channels to 4.
Then I put a Native Instruments B4 Organ as my first VSTi in the FX bin and set it to output on all 4 channels.
Next I added a Pantheon reverb as the second item in the FX bin and set it to only receive on 3 and 4, but to output on 1, 2, 3, and 4.
Lastly I put EZ-Drummer as the last VSTi in the chain, outputting on 1 and 2.
End result was the B4 Organ has reverb on it, but EZ-Drummer does not, and it all lives on one track.
Edit:
All that said, I'd never do it like that if I wanted BOTH to use multiple VSTi's that play from a single midi track, AND be able to individually add FX to them. If I were to make a piano/organ/strings/horns thing, I would put the piano, organ, strings, and horns each on their own track, have another track that records the midi and routes it to the four VSTi tracks, and a final folder track that the monstrosity instrument sound all lives in that could be collapsed down to a single track on screen when not making adjustments, but can be opened where everything is easy to see and make changes when need be. That's my $.02