Yeah, you can't have two GCD spells on one macro without using /castsequence or a modifier, because they don't want you macro'ing your entire rotation.
You can reset castsequence with shift/alt/ctrl, but you'd be better off doing this:
Code:
#shotooltip
/cast [modifier:shift] Hand of Gul'dan; Shadow Bolt
Or, if you don't mind hovering your mouse over the enemy as an action to allow you to cast both spells on one button, then this one should work:
Code:
#shotooltip
/cast [target=mouseover,harm] Hand of Gul'dan; [harm] Shadow Bolt
(Note: I'm not sure mouseover works for AoE spells, I just tested these with lightning bolt / chain lightning to see if they work.)
This makes it so you if have a mob targeted, it will cast shadow bolt, but if you have you mouse hovered over a harmful target and press it, it will cast hand of gul'dan.
-----------------
Either way you cut it, you're going to have to execute an additional action when dealing with multiple GCDs, be it holding shift, hovering you mouse over them, or simply clicking two different buttons.
Edit: Testing on my mage to see if it works with AoE spells.