The actual "rules" will of course depend on that country's regs. However,
AFAIK, most would say the 1st flight should be solo.
I think the FAA are fairly keen about most of the test flying being solo? In the UK, the test phase of an RV only needs to be "5hrs or more", and the LAA emphasise a suitable observer is "desirable" for much of the Test Phase. However, they do say Flight #1 is to be solo.
I've done 2 RV-8 1st flights for other owners, and think the limited expectations / ambitions for the 1st flight do not require an observer.
NoD