Using dual RCD boards you would be limiting disruption:
RCD 1
Sockets Downstairs
Lights Upstairs
RCD 2
Sockets Upstairs
Lights Downstairs
If one of the RCD's should trip and not reset due to a N-E fault then you would have provision for lighting on either floor via lights themselves or table lamps via sockets.
As you say though, RCBO's improve it even further.
RCD 1
Sockets Downstairs
Lights Upstairs
RCD 2
Sockets Upstairs
Lights Downstairs
If one of the RCD's should trip and not reset due to a N-E fault then you would have provision for lighting on either floor via lights themselves or table lamps via sockets.
As you say though, RCBO's improve it even further.