landing lighting trips circuit breakers?

hi - we have replaced an aged old dis board with a new modern consumer unit with split RCD's. The lighting is set up so that the ground floor lights are on one side of the board i.e. protected by one RCD, and the first floor lights are on the other side of the board i.e. protected by the other RCD.
Having replaced the board, I've just started putting new light fittings back in (we'd taken the old ones off before the board was replaced). All the lights work (i.e. both circuits are fine) but the 2-way switch first floor landing lights keep tripping the whole board.
I’ve double checked all the connections, and I’ve realised that the first floor landing lights are effectively taking their positive feed from the ground floor circuit, and the negative feed from the first floor circuit – I’m hoping that this is the cause of the problem i.e. you can’t have circuits joining in this way!!

Any advice greatly received!!

cheers