In my setup (Cubase Pro 12), this results in identical sound between the stereo and dual-mono configurations.
I think it might be a problem with nuendo not able to measure negative values on some soundcards like RME. Found that it is a known thing adressed in this forum saying :
âSo it seems ⌠cubendo is incapable of accurately reporting delay on external effects with certain hardware.â
Measure the delay manually by recording a transient signal and inspecting the delay on the ruler, then enter the value in the external plug-ins manually. Itâs a one-time affair, after that youâd be all set.
yes but unfortunatly no because itâs a negative value 
I did the measurement itâs 50 samples whatever the buffer.
this helped me to set the record shift = -50samples.
So when I record the send returning from my hardware on an audio track (and not using external plugin option) after I pressed Stop the waveform aligns perfectly with the source signal.
But with external FX on the insert you canât add a negative value and thatâs why âmeasure delayâ is not working.
Thatâs an additional layer of complexity youâll have to deal with. However, it applies equally to your stereo and dual-mono configurations and thus cannot explain why youâre hearing a difference between the two.
And thatâs a great idea! Iâll do that.
Thatâs even better then. I like turning on gear at the start of a session and know itâs working as it should. Resolving my issue with dual mono external gear has streamlined my workflow beautifully.
Oh, donât tell me that! I was thinking how extraordinary your hearing must be to pick up on such a small delay. 
You donât have to create 1xStereo and 2xMono, you only need 1xStereo and 1xMono.
When you want to use the external effect as a stereo instance, connect L&R to the stereo, thatâs usual. Then when you want to use them as dual mono inserts, disconnect only the R channel from the stereo instance and connect it to the mono instance.
i.e. You can call them âStereoâ and âMono Râ whereas âStereoâ can be used as âMono Lâ.
If you insert the stereo instance into the mono track, only the L channel will be processed anyways, so you can disconnect the R channel and use it as a mono. Also, you only have to re-route the R when you want to use the 2nd mono instance. So this way you donât have to re-route that much as you do it with 3 instances.
Iâm one of the people who used to find this issue quite inconvenient as well.
I recently upgraded from Cubase 12 to Cubase 15, and I noticed that when multiple buses are created, it is now possible to assign the same input/output channel to different Device Ports. In other words, overlapping assignments are now allowed. As someone who regularly uses some hardware effects, this is a clear improvement for me!
This was not possible in Cubase 12. Thatâs why Iâm curious about when this change actually happened. Does anyone know in which version of Cubase this improvement in assignment flexibility within Audio Connections was introduced?
As far as I know only the default setting was changed. There was an option to allow multiple busses to be using the same audio device for a long time. Before C14 this was disabled by default, since C14 it is enabled by default.
Thank you for the valuable information. I looked into it, but I couldnât find that option in Cubase 12 or Cubase 15. I wonder if an option that existed in Cubase 14 was deleted in Cubase 15?
Itâs in Preferences â VST-> Control Room, but it frees up only devices that are used for the monitors, not cues.
Indeed, in Preferences > VST > Control Room, there is an option called âExclusive Device Ports for Monitor Channels.â
However, after testing with this option both enabled and disabled, I confirmed that in Cubase 12, assigning the same device port more than once still triggers a warning, as shown in the image below.
The selected device port is used exclusively. This connection will be ended. Do you want to continue?
In Cubase 15, regardless of whether this option is checked or not, duplicate device-port assignments are now allowed without showing the warning seen above.
In any case, thank you very much for your very interesting advice. The fact that Cubase 15 now lets me use a stereo hardware EQ as two mono hardware EQs without reopening the Audio Connections window is a significant improvement for an outboard user like me.