"Lurch" wrote
| Of course, the latter would mean that going to at least one
| light switch would be power from both up and down lighting ccts.
| Is that a problem?
| Technically, yes. You should keep different circuits out of switches
| if possible, although 90% of the homes in the UK will be wired with
| upstairs and downstairs circuits connected at the downstairs switch.

To be precise, the two circuits should be kept entirely separate
*electrically*.

I see no reason why two switches on separate circuits should not share the
same faceplate (in a single-phase installation). It would be advisable to
place a warning label inside the switch box though.

Owain