Nothing wrong with it, but if you put one in higher up and 'loop' the supply, you will end up with two 6mm² T+E's, plus the cable to the hob, so no saving on space? And if you took two separate 6mm² T+E's upto the cooker switch, again, you have three cables to terminate.
You can actually usually use 2.5mm² or 4mm² H07-RNF flex (or equiv)(depending on loading) from the oven/hob to the outlets, and this is actually specced in most manuals these days. Nice and small, and easy to terminate.
You can also get "Dual Cooker Outlet Plates"

Google gives them here:
http://www.alertelectrical.com/prod/1022/click-45a-easyfit-dual-appliance-outlet...Nothing wrong with coming out of your existing plate and into a JB, splitting to the two appliances either. You would use a 'coffin' JB.
http://www.lamppostelectrical.co.uk/products/mounting+&+junction+boxes/junction+...