There is no technical reason why it wouldn't be possible to have 2 vRA environments connected to 1 vCenter.
The only thing to consider is that it would be better not to mix the VM's (e.q. do not modify one VM from both vRA's).
Regarding downtime, it is not an upgrade but a migration.
During the entire migration period, you can have both vRA's up and running.
That is, if you are careful when doing the final onboarding properly.
About vRO, the source vRO will go down for a few minutes at the point where you run the 'vro-migrate' step.
That is when you copy/clone the entire vRO content from vRO7 o vRO8.
My experience is that that will only take minutes and after that you can again have both up.
I am not sure what would be a limitation with external vRO, I guess that does not matter.