I'm trying to patch multiple soft synths to a single midi track and can't figure it out.  I'm new.  The midi track only has a single output.  How can I assign multiple synths?

There might be other ways but the simplest way I can think of is just insert new tracks for each synth you want in the project then copy the MIDI clip into those tracks.
 
 

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.

I suspect the "copy the data" is the only way
but the thought of this makes one wonder why is there not a virtual midi patch bay.

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.

hmm... detune? If you just want to have some octaves or fifths or whatever just copy the MIDI notes and lower/raise them.
 
As far as "overhead" it's not really a big deal if you have a decent computer. This type of layering is exactly what a multitrack program is supposed to do. What will really suck up resources is the synths themselves. The MIDI data is no big deal.
 
If you are using a multi timbral synth you can send multiple tracks to it through the various synth channels so you can play multiple instruments at once while only using one synth. I think there is a tutorial showing how to do this with TTS-1 at the start of the manual... not sure though.

One way is to use a virtual MIDI cable. LoopBe1 and loopMIDI are free. Route the incoming MIDI data to a MIDI track setting the output of the track to the virtual MIDI cable. Then set the inputs of the synth instrument or MIDI tracks to the virtual MIDI cable.

Yes, I looked for a patch bay plug-in and couldn't find one.  Even being able to do a midi pass through from one midi track to another would be an option.  Though there would still be multiple midi tracks at least you wouldn't have to put the events into each track.

I'll check out the virtual midi-cable; sounds like an option.
 
Beepster - on the detuning, I'm just detuning a few cents.  Basically like double tracking or when you tune a piano. You don't tune all three strings exactly the same.  Just being a tiny bit off really fattens the sound.  (I was also looking for a plug-in that dynamically and randomly shifts the freq and timing just a touch to emulate double tracking)
 
The overhead was on me, not the PC  - I'm too lazy to copy and paste!  My PC is pretty massive.

Tessalator
I'll check out the virtual midi-cable; sounds like an option.
 
Beepster - on the detuning, I'm just detuning a few cents.  Basically like double tracking or when you tune a piano. You don't tune all three strings exactly the same.  Just being a tiny bit off really fattens the sound.  (I was also looking for a plug-in that dynamically and randomly shifts the freq and timing just a touch to emulate double tracking)
 
The overhead was on me, not the PC  - I'm too lazy to copy and paste!  My PC is pretty massive.

Hmm... well a lot of those synths are tweakable so maybe you could fiddle with them to fatten things up and you may be able to use the multi outs on some of those synths to just use different variations of the same patch (I don't know enough to say if this is possible nor would I know how to set that up).
 
However as far as the copying you could just Right Click/Clone the track and it will set the whole thing up for you again (and I think create a new instance of the synth). Then just change the synths settings on the new tracks. If resources are no issue then it should be fine.
 
I'm not a proficient synth guy yet though so those are mostly theoretical guesses. Cheers.

And to make things a little easier put the clones in a folder with original track and set your Comping mode record settings to "Group Clips In Folders" so that way when you edit one track's clips they all change. Pretty much eliminates all the problems of dealing with multiple tracks. You can group the clips manually retroactively as well by selecting all the clips, right clicking and selecting Group Selected Clips or whatever it's called.
 
So a bit of work at first but easily doable.