You need to know what your total amp draw is. If your combined amount is 80% of the rating of your relay (I like to have some extra overhead for surges), then it really doesn't matter what you hook up to it. If you had 20 lights that pulled 1 amp each, you can run them all off one relay but if you had 2 lights pulling 10 amps each, that would be your limit. The relay doesn't care how many of anything, just how much is being pulled through it.
If you want the new lights to be independant of any other lights, then a second relay and switch would be needed.
In case you don't know how to figure out your amp draw, take your watts and divide by the volts to equal amps. Even though your running voltage would be typically 13-14 volts, I always use 12 volts in the equation. Below that, you shouldn't be running anything extra. Make sure you wiring and fuses are set to handle total load too.
Good luck.
