ideally you would use 2 different relays for the same switch. even though you are under the amperage rating, heat can play a role in it. take the desert for example. the relay is an electromagnet that when triggered closes the circuit. the opposite to that, is when the relay is triggered, heat is produced as a biproduct of the mechanism. if you have 2 lights on in 100+ weather, both on the same switch, very close to the maximum amperage rating of your relay, you could get too much heat generated in the relay causing it to fail and possibly having no lights whatsoever, or blowing fuses. but this is an extreme case and most likely will not happen, however i have seen these types of issues before. you may never have a problem with it, as others dont have this issue. but i figured the info is pertinent to your case