Looks like a similar setup to mine, assuming it is an alarm. I've got a Toad unit that has a central locking output. Can't recall whether it has separate L and R indicator drives. You could as @Gooders says use two diodes to isolate the L and R drives, but they'd have to be rated to handle at least two 21W bulbs. A 5A rating should be fine. Or use a double-pole relay to switch them, where the relay provides the isolation. Another poss is to look at the wiring diagram and see where the emergency flasher switch powers both together - from memory, this is how I did mine, so both pairs flash at once. The flasher switch itself is the best place to hook into this stuff, as everything's neatly labelled.