I know with my sister house a little on the large side the sockets were split left to right as that allowed a better loop impedance, I can not actually see any good reason to split wiring between upper and lower floors other than easy to label. If using a two RCD consumer unit you would want to avoid in an emergency running an extension lead up or down stairs, so safety wise you want a supply from both RCD's on each floor. Where the problem arises you also want every room to have the lighting and sockets from a different RCD as well so should anything plugged into a socket develop a fault it will not also plunge one into darkness.
I really do query the whole idea of only 2 RCD's in a consumer unit? It is hard to arrange the wiring so lights and sockets are always from different RCD's in every room, however it does not matter how many RCD's a general power cut will still plunge one into darkness, and even the street lamps will not help, they will go out too.
I think every house should have the risk assessed. My house has the stair case in the centre of the house, there are panes of glass above every door on the landing to allow light from bedroom, bathroom and toilet to reach the landing, but I decided the risk was too high and fitted an emergency lamp above the stairs.
Over the years the house has changed, this includes ripping out the cistern and fitting an instant gas boiler in the garage, this resulted in no immersion heater and the cable was used to power sockets in kitchen, living room, and the bedroom made from room which did have cistern, when the kitchen was extended two radial circuits I had fitted were combined to make a new ring final, I also have a dedicated supply to cooker, so in the kitchen there are 4 MCB's protecting the sockets.
In living room 2 MCB to sockets, maybe 3 and same in master bedroom and bedroom formed from airing cupboard and front bedroom. With exception of the socket on the cooker supply there are dots on the sockets red, yellow and blue and dots on the consumer unit so I know which MCB feeds which socket. Or at least there were, but cleaning has resulted in some now being missing.
I am sure most houses have something which is not powered from either of the two standard ring finals, so it would be crazy not to test each socket before working on it, in one house I even found a socket powered from next doors supply, so even turning off the isolator is not enough.
I do not always use a proving unit, but I do test for dead with a non switched tester which does not require batteries so very unlikely to be an error and also use a neon screwdriver in case there are any errors like a borrowed neutral, I know today with clamp on ammeters you can test for borrowed neutrals before you disconnect but by using a neon screwdriver the risk is low.
Even some of the proving units do not really ensure the tester is OK, the proving unit should go up and down the voltage range to show the tester will show 230 volt or in fact any voltage over 50 volt, however the one I used gave out 500 volt, so although you knew all lamps would light at 500 volt you did NOT know if any light would come on at 50 volt, so rather a useless proving unit.
Although you can get locks to fit MCB's, and in some cases you can lock the flap on the consumer unit so no one can switch a circuit back on, in domestic the electrician often does not even place a sign on the consumer unit or even tape on the MCB. I know tape is not really good enough, but even locks can be removed, a tie wrap is good enough, it needs a tool to remove it, and main point is to ensure anyone who does switch it on, knows full well some one was working on the circuit. Electrocution is a valid method for murder, but we don't want it to happen by accident.
Be it bolt cutters to take off a lock, or master key, a pair of snips to remove a tie wrap, or a simple bit of insulation tape, even a notice do not switch on, some thing should be put on the consumer unit to show anyone trying to locate a fault that it's turned off intentional. I have had it where an electrician got a shock from an isolated supply where another electrician was looking for a fault using an insulation tester. Also where proved dead then a contactor came in causing it to become live. Even trying ones best things can go wrong. But not actually testing which MCB covers which circuit at best is negligence, I if I was caught not testing first I would expect the sack.
Not testing first is gross negligence so no need even to give a warning, it is instant dismissal. Or of course death!
