Any external pir will do, no slave needed, both sensors just pick up movement and switch a 'live' onto the switched live conductor. The cabling just 'daisy chains' from the feed to the first sensor to the first light, second light, third light and on to the second sensor. At the feed end you connect a live/neutral/earth and terminate the un-used switched live conductor - then at the first sensor you connect all conductors as per the diagram for the sensor, at the first light you connect switched live/ neutral/earth and join the two live conductors to eachother so they are continuous, repeat at other lights and finish at last sensor connecting all conductors as per sensor diagram (live/neutral/earth/switched-live)