If the previous spell has a buff, you can Dummy the ability.
Make an ability that does nothing, base it of rejuvenation, for example. Then make it so you can cast it on your enemies. When you cast this spell check if the unit has the required buff or not. If it has, create a dummy unit and order it to cast the spell, if not, nothing happens.
If the first ability doesn't give a buff, then it's a bit more complicated i think x)
What he's saying is that you make the spell have a 'Requirement', in form of a unit, that unit is a dummy unit that is created when you cast the first spell. You might want to add an expiration timer (timed life) to the dummy. Name the dummy unit '(FIRSTSPELL) Cast', this will display;
Requirements
- First Spell Cast
When you cast your second spell, remove the dummy unit, thus disabling the ability and shows the DISBTN icon (the dark, faded out one).