Tessalator
I've been doing the copy and paste, but it seems like a lot of overhead.  I'm just trying to layer and detune a few synths to get a fatter sound.  Seems like something that would be a fairly common need.

Hi, Tessalator - As usual on this busy Forum, you've gotten some very helpful replies.  I'm adding my 2 ¢ because I think it may be helpful also.
 
You're right that what you want to do is a fairly common need, and the guys are right that you could deal with virtual MIDI cables and set up what you need - But that's not how people usually accomplish what you want. 
 
You said that you think C&Ping tracks is "a lot of overhead" - not sure what you mean.  That's very simple, and MIDI tracks don't need more than a whisper of power to be in your project file - it's the synths themselves that need the power, and at some point of course you'll hit the ceiling of how many you can have churning away at the same time.
 
But that's the best method, IMHO, to simply copy the mother track and re-direct the copies to the synth you want to have layered in with the first one.  Back when we only used hardware synths, layering modules was what the whole MIDI thing was about, detuning each one and getting that big wall of sound - The process was a bit different then, but that's the sort of result you're talking about.
 
If you want to layer various patches from one multi-timbral synth/sampler, that's even simpler.  You just set all the available slots (usually 16 on a multi-timbral unit) to the same MIDI channel, and then tweak each patch as needed, as long as you don't automate the tweaks, since all those loaded instruments will be reading from a single MIDI track.
 
So - Copy tracks - That's the quickest, most efficient way to do it.  Assign instruments to the same MIDI channel when you're using multi-trimbral plugins with instruments you want to include in your layers.

Randy

Thanks all.  You have been very helpful.  rbowser, yes, that's what I'm trying to do - the old approach for external synths.  With my TX816 I do like you say, just assign each module to the same midi channel.    I was looking at the TTS but didn't see how to set the midi input channel for the individual modules.
 
I did find a way that works starting with Beepster's grouping clips suggestion.  If you clone a track there is a checkbox for "link to original", and a number of copies option.  I just did a clone of three all linked to the original and changing the original changes all of the clips.  It gives me the sound I'm looking for with the approach I'm used to (I'm just now coming out of the 70's and moving from my 3340S 4-track tape to digital.)  To save CPU I render the tacks to an audio track and then mix them into one. 
 
I don't know if I should start a new thread, but I have a related question for this approach.  Is there a way to disable a synth but keep it in the project?  Once I have the audio I don't need to run the synth, but if I need to re-render I would need it again. 

On each track you will see a button that looks like a snowflake. That is the "Freeze" button. Pressing this will "freeze" the track which is basically a temporary render that bounces any synths and effects to audio and disables the synth effects thus freeing up resources. You can click that button again to "unfreeze" it and now you can edit the synth/effects again. You can also just freeze the synth without freezing effects and vice versa as well. Look up Freeze in the manual to learn more.
 
Cheers.

Tessalator...I was looking at the TTS but didn't see how to set the midi input channel for the individual modules.
...Is there a way to disable a synth but keep it in the project?  Once I have the audio I don't need to run the synth, but if I need to re-render I would need it again.

In TTS, right click while hovering over the fader just to the left of the loaded instrument's name.  You'll get a "control change assign" option, including what MIDI channel you want that instrument slot to respond to.
 
In the browser, with your list of synths, click the "un-dock" option and you'll see buttons appear to the left of each synth.  Clicking those either disengage or re-engage the synth.  Then dock that synth rack again when you're through.  It looks like they made those On/Off buttons only appear when you un-dock in order to save screen space.
 
Randy

Sorry... I should have mentioned you can Freeze a synth directly in the Synth rack browser too without freezing the actual tracks. Same benefits.

Cloning the midi track is good for changing notes,... for each synth
 
For unison, full big sound, use a midi cable, virtual, or real. 
 
I Have used both.   I have loopmidi and loopbe installed.  Follow the links provided by Scook and you will be set for future routing.
 
If all the synths are to be unison, place the synths output tracks in the same folder, and send to the same sub-bus, and then send to the All Instruments bus, and then to the master mix bus.

You guys are awesome, great forum.  Thanks.

rbowser
In the browser, with your list of synths, click the "un-dock" option and you'll see buttons appear to the left of each synth.  Clicking those either disengage or re-engage the synth.  Then dock that synth rack again when you're through.  It looks like they made those On/Off buttons only appear when you un-dock in order to save screen space.

Or dock the rack in the Multi-dock. The buttons are visible there too.
 
Another option would be to bounce the audio to a new track and archive the synth using the "A" in the track header. Freeze performs an in place bounce and archive in one step. If the track is unfrozen any changes to the audio will be lost however the frozen audio can be dragged or copied to another audio track to prevent loss.