You can, but I'd rather not for a couple of reasons.
1. Many PIRs have functions accessible by switching on and off with certain timings, but if two lights get out of step that can be a pain.
2. When one of the fittings develops a fault keeping them seperate lets you isolate it and keep using the other one.
Getting on with the now. Spurring off a double socket that's in a radial line. So the socket already has supply going into it and supply going out of it. If I spur off this socket which has two earth terminals (one has the IN, one has the OUT cable), so what happens to the earth wire in the spur to be attached?
I can't decide if it matters where it goes, but I wont be guessing.