Many landing lights can be switched with two switches, and years ago it was common to use two gang switches and borrow the line from the other switch, this was OK if both lights used the same MCB, but then people would separate upper and lower lights, once separated you need three cores between the switches, and you can't borrow the line or you get a borrowed neutral situation.
If the two MCB's come from same RCD it is still wrong but it works, but if different RCD there are 4 combinations of the two switches 2 being off, one is on and will not trip lights, one is on and will trip lights.
It may be some thing completely different, but two RCD's tripping rings alarm bells, and would look at landing lights.