It's the amps that govern wire sizes. Check the lights for the amp ratings. At 70 watts they should draw around 12 amps. If so, then 18 gauge wire is fine for the power circuit if you keep the wire length under 10 feet. If over 10' then use 16 gauge. I always use relays for my wiring so I don't have to have large switches in the cockpit. The trigger circuit of the relay can use very thin wire, like 18 or 20. That said, if your switch is rated higher than the amps of the two lights, you don't need a relay. Make sure you fuse it accordingly.