Dual RCD seem to be linked and only one stays on. Although the whole house has power

It could be a shared neutral between two circuits. That would be a cause of only one RCD being able to stay on at a time. However if your retaining full power to all circuits when one is down then you have a deeper problem. If you had two interlinked circuits for instance including a live conductor then potentially this could supply the circuits on the down side RCD with power. One thing I would quickly try is turning each circuit breaker off individually on the down side of the RCD and seeing if this A. disconnects the circuit it is protecting and B wether it disconnects any other circuit. If you find one of the circuit breakers disconnects another circuit or several than that is the circuit with the problem. Just a theory anyway! You definitely need a trusted electrician on site to resolve the problem though.

Have you recently had the consumer unit upgraded? A mixed neutral would manifest itself straight away? Dual RCD (RCCB) consumer units need to be wired so that circuits aren't mixed between the two RCCBs. By that I mean the neutral and phase would be on the same side and in the same corresponding L and N for that circuit. However, this type of fault would not allow you to set one or both RCCBs, so, unless its a replacement consumer unit and the electrician mis-wired it, I doubt this would be the cause of the fault. Can you post a photograph of the consumer unit and the problematic RCCBs?

Could be that you have a shared neutral on a lighting circuit

More likely to be borrowed neutral to upstairs and downstairs lighting in my experience