Summing stack and external midi device?

I ran into this as well.  I discovered that you first have to make MIDI tracks for internal virtual soft synths (should not matter what the instrument is), e.g., if you want to end up with 2 external synths in a summing stack, then first create 2 internal virtual soft synths, select those 2 tracks, and make a summing stack with them.  Now (after the stack is made), expand the stack, then go and re-assign those tracks (right click on each track to re-assign) so the synths are now your external synths (however you have defined them, e.g., maybe you have made them using the external synth plugin).

Maybe the bad news about this workaround is that I am not sure that external synth tracks in a summing stack work the same way as when they are just in the arrange window like any other track.  For example, if in Project Settings -> Recording you have "Auto demix by channel if multitrack recording" - this option does not seem to work anymore once the external synths are in a summing track.    Or, multi-channel MIDI data is no longer split out to each external synth track with a matching MIDI channel number after recording (in this case each external synth track assigned to an individual MIDI channel).  But this does work if you put the external synth tracks in a folder - the only option in the "Create summing stack" window for external synth tracks.  (And it works with the ext synth tracks in the arrange window like any other track).

I had hoped that you could put external synths in a summing stack, arm the stack to record, and then recording ON the summing stack would work the same way as it does when they are separate tracks in the arrange window (each armed to record).  But no such luck.  And unfortunately, a folder stack does not even have a record button.

are summing tracks possible or not with external midi devices?

No. Summing stacks "sum" (mix) audio signals. An external MIDI track doesn't output an audio signal, therefore there's nothing to sum. 

What are you trying to do? 

If tracks output audio, then it should not matter if they contain references to audio or MIDI objects; it should work to create a summing stack containing them.  You should be able to create a summing stack that contains both audio and MIDI tracks (that output audio). 

Selected virtual instrument tracks will get their audio outputs changed to the next  bus that Logic assigns to the input of the new summing stack track (just like a selected audio track would).  If you record MIDI on the summing stack track, then the virtual synths in the stack will all play that same MIDI data (which is convenient).  

I was just able to create a summing stack for external synths via tracks that use the external instrument plugin (which also outputs audio on its track).  Not sure why I was not able to do that directly per my previous post.  But I was used to being able to use “Auto demix by channel if multitrack recording" (i.e., MIDI multitrack recording) - which does not work for me in a summing stack.  

If tracks output audio, then it should not matter if they contain references to audio or MIDI objects; it should work to create a summing stack containing them.  You should be able to create a summing stack that contains both audio and MIDI tracks (that output audio). 

Correct. Using Logic's terminology, there's a distinction between Software Instrument tracks (that host MIDI regions but output audio signals) and external MIDI tracks (that host MIDI regions and output MIDI data).