In my living room I currently have a single light which is controlled by a
single switch.

I have put in a number of halogen lights which now all trigger off the
single switch

I am looking to put some X10 control onto these lights such that there are
two circuits: Circuit 1 controls 3 lights & Circuit 2 controls 2 lights.

I am more than happy about how to convert the single circuit to 2 with the
X10 moules turning on the lights but I would also like the old light switch
to still work i.e. flick the old light switch and it turns on both circuits

The bit I am struggling on is how to wire this is that if you put the switch
to both circuits then it will cause a short on the X10 modules thus turning
all lights on when either X10 circuit is switched on.

Is it just a case of using the switch to turn on relay triggered feeds (thus
isolating each circuit? If so would some normal 240V 3A ones be OK? i.e.
http://www.maplin.co.uk/images/Full/37495i0.jpg The only limitation with
this is you could not have them dimmable.)

Is there a way of doing this keeping the main dimmer with the capability of
dimming all lights?

[2 Light Circuit = 100W]
[3 Light Circuit = 150W]