You mean a macro to link these two spells together? That's what I'm getting from this.
If that's the case, I don't think you can macro them together so they both cast at once since they trigger a global cooldown. You'd have to hit the button twice and even so, this macro seems so strange and almost pointless if I'm understanding your question.
I'm aware of the global thing, I just wondered whether there was an option. With regards to it being (in your opinion) strange and pointless.... Don't let it bother you, it doesn't hurt to ask.
I just want to ensure that Ice Barrier is up all the time. Seeing as the fireball spell is a filler one, I thought it best to attach it that to ensure maximum up time. But, it seems that it's a unlikely proposition so.....
YOU CANNOT CAST FIREBALL AND ICE BARRIER AT THE SAME TIME, EVER
You can only make them mutually exclusive
#showtooltip
/cast [mod] Ice Barrier; Fireball
that will bind both abilities to one button, but you can only cast ice barrier while holding either Shift, Alt or Ctrl, and only cast Fireball when NOT holding Shift, Alt or Ctrl
if you want maximize up time, use an addon like Power Auras to make ring a bell whenever Ice Barrier falls off, you will get in the habit of reacting immediately to the bell, and will naturally approach 100% up time on ice barrier.