/castsequence Unleash Elements,Lava Burst

This should cast Unleash Elements on the first key press, then Lava Burst on the second, but I assume you want to cast whatever is not on cool down?

/cast Unleash Elements
/cast Lava Burst

Will do that for you, but you have to take into account the GCD which will prevent the Lava Burst from being cast if you press the key before the GCD is over. Most MBers use these types of macros to handle this sort of thing. Altho it seems to me that it doesn't work 100%, seems to me that you might have to "spam" the key a bit, but haven't done any extensive testing to check this TBH. I've just noticed that it works, so haven't examined the issue a lot.