You can run things simultaneously if they do not trigger global cooldown; one example would be Lithy's PoMPyro Macro:
/cast Presence of Mind
/stopcasting
/cast Arcane Power
/stopcasting
/use 14
/stopcasting
/cast Pyroblast
The "use 14" is my on-use bonus spelldamage tinket, and the "stopcasting" between each action ensures that the next action can take place. I do not know if the stopcasting is needed anymore with 2.3, but my macro still works, and I won't interfere with it.
It's worth noting that with a little intelligent macro'ing, you can as people above have said make it so that click one will cast the first spell and click two will cast the second. It's also possible to script it so that the button will give different effects depending on whether you left, right, shift-left, shift-right, alt-left, alt-right, ctrl-left or ctrl-right click it.
If you want to know more, there's a fairly decent macro guide on WoWwiki (off the top of my head I think it's
THIS page).