Because I do know everyone loves a good diagram. 
Does anyone know where, apart from Argos sell double curtain rails.
There's a longish bracket with two grooves (?) to take both a net or voile rod and a second to receive a curtain pole.
Of course, the ones I want (black nickel) are on clearance, so tomorrow I will be shlepping across the West Mids to find the last two.
Also, the reviews aren't exactly glowing.
The complaints are similar, that the brackets couldn't withhold the weight of the nets, the two poles and medium weight curtains.
Does anyone else sell them with stronger brackets, or do I have to hit up eBay or Amazon?