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 :
“So it seems … cubendo is incapable of accurately reporting delay on external effects with certain hardware.”
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. It’s a one-time affair, after that you’d be all set.
yes but unfortunatly no because it’s a negative value 
I did the measurement it’s 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 can’t add a negative value and that’s why “measure delay” is not working.
That’s an additional layer of complexity you’ll have to deal with. However, it applies equally to your stereo and dual-mono configurations and thus cannot explain why you’re hearing a difference between the two.
And that’s a great idea! I’ll do that.
That’s even better then. I like turning on gear at the start of a session and know it’s working as it should. Resolving my issue with dual mono external gear has streamlined my workflow beautifully.
Oh, don’t tell me that! I was thinking how extraordinary your hearing must be to pick up on such a small delay. 
You don’t 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, that’s 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 “Stereo” and “Mono R” whereas “Stereo” can be used as “Mono L”.
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 don’t have to re-route that much as you do it with 3 instances.