Hey Adrian

To me, analog summing would be to route single tracks from Logic to their own dedicated analog outputs and from there to a summing mixer which creates a stereo mix out of the single signals the analog way.
But as far I understand it, the signal summing you do within Logic, right? Then adding some analog processing to the mix?
Anyway, the output you choose in the I/O plugin is equivalent to the channel in the 'software playback' row in TotalMix - NOT 'hardware output'. To have the signal on the physical analog outputs, you have to click on the corresponding fader in bottom row - 'hardware outputs' - then open the 'software playback' fader (and also it's important to close all other faders because otherwise you could accidentally have unwanted signals from hardware inputs or software playbacks on this output channels).
Side note: That would say, you could theoretically choose outputs (for ex) 5-6 in Logic/plugin but route this to physical outputs (for ex) 7-8 within Totalmix. For the Mac / Logic DAW software it would still be outputs 5-6 because the 'detour' is made within the audio interface. Of course you would have to plug the cables in outputs 7-8 on the interface then.
You know, the app 'Totalmix' is to control what happens within the interface's DSP. So what you see in the Totalmix window, doesn't happen inside the computer but inside the interface.

In the I/O plugin you then choose the inputs where the analog units are connected on the UFX+ physical inputs. So the signal comes back to the software mixer within Logic.

First time I used exactly this feature, my head was really smoking...

Have fun!