Oh, so close to not being a dope but I didn't quite make it.
 
It turns out that you get two P&M Chainer plug-ins when you buy the product, one is effects and the other is a VSTi for instruments. I had what I needed all along!
 
The instrument one works a treat. Load it up like a synth and start stacking. It has some very nice capabilities--you can do some mixing, panning, FX, etc. And one MIDI track to rule them all. I thought it was money well spent when I thought it was only for effects! I still think Cakewalk should be mindful of what the competitive DAWs are doing, but in the meantime this will do very nicely.

KeithAdv
Oh, so close to not being a dope but I didn't quite make it.
 
It turns out that you get two P&M Chainer plug-ins when you buy the product, one is effects and the other is a VSTi for instruments. I had what I needed all along!
 
The instrument one works a treat. Load it up like a synth and start stacking. It has some very nice capabilities--you can do some mixing, panning, FX, etc. And one MIDI track to rule them all. I thought it was money well spent when I thought it was only for effects! I still think Cakewalk should be mindful of what the competitive DAWs are doing, but in the meantime this will do very nicely.

I would stick to creating the separate instances of the synths inside Sonar.  In the past I have used Bidule and FLStudio as plugins inside Sonar and now after upgrading I no longer have the machines set up in the same way so I have lost the config for the plugins. So if I want to work on those projects again I have to rebuild everything. Luckily I don't need to revisit those old projects very often. 

I've always found that the added flexibility I get from having separate MIDI tracks to control separate intstruments is worth more than whatever convenience might come from separate MIDI outputs. Sure, I might want to stack sounds to create bigger or more complex sounds, but what if I don't want them stacked all the time? What if I want the second layer to come in later for bigger effect, or drop out to bring things down? Easier/more flexible to have them visible on separate tracks to see what's happening where and when. If I really need to get them out of my visual space I can always just put them in a folder and collapse it! 

Having said that there are free alternatives to Chainer that can accomplish the same result. I've played around with Imageline Minihost, which is beta and possibly abandonware at this point but seems to work pretty solidly from my limited experimentation. And can also be used, in a pinch, to do all sorts of wacky/creative things... don't want to post links to competing companies here, but a quick search should turn it up for those interested. 

KeithAdv
OK, so the answer is no, Sonar doesn't have a convenient way to stack synths. That actually makes me feel better, because at least I'm not a dope!
 
I use linked clips sometimes, but that is a workaround in this case and can be a tedious one at that. It would be wonderful if you could just drop another send into a MIDI track. That capability instantly suggests creative possibilities to me. Studio One's Multi-Instrument mode is a dream. Hopefully something similar is on someone's to-do list at Cakewalk.
 
Thanks folks!

Currently, the best way to stack synths in Sonar, without copying midi tracks around, is to use a multi-channel synth that can stack synths internally, like Omnisphere, or to use a VSTHost like Blue Cat's PatchWork or Vienna's Ensemble.
 
Dan

If you happen to have Fishman TriplePlay, that plug will work with any MIDI input.