Update of my experience:
Today I received my new Pixel 8 Pro.
Have tested it first with stock rom Android 14 and tried to setup two eSIMs for parallel use …
Conclusion: it was easy to do, no error message like before with my Pixel 7 Pro under GrapheneOS, but …
After flashing GrapheneOS the eSIM at #1 was no longer available!
Only the eSIM installed first was visible at #2!
Then I have installed sandboxed Google Play services to enable privileged eSIM management,
after a reboot there are both eSIMs again with the names I had given them under stock Android 14. :-)
But I was afraid what will happen, when I disable privileged eSIM management again…
Because I want to use this Pixel 8 Pro without Google Framework Services…
Have tried it, because it is a fresh installation of GrapheneOS, nothing of my stuff installed yet, so it wouldn't be much waste of time, if it doesn't work as expected…
Conclusion: Both eSIMs are still visible with the names I have given them and are fully usable, without any privileged eSIM management and without Google Services. :)
P. S.: So there's something wrong with my Pixel 7 Pro or maybe there's a series that simply can't do dual eSIM yet? Very mysterious, but the main thing is that my new main device can. :-)