Do I need to have these done at the same time or may I have the car aligned and corner balanced at a later date?

corner balance first.
then align



corner balancing is about changing individual corner's ride heights to achieve balance.

but changing ride height also changes alignment

if you corner balance later, you have to align again to be spot on.

And if you don't have adjustable perches in the rear, you really can't corner balance it.

One problem with 914 rear suspension related to this is that changing the rear camber will change the ride height by margins enough to mess up a good corner balance. You'll need to get the rear camber reasonably close to where you want it during the corner balancing process. The other measures of alignment will have much smaller effect on corner balance to the point of being negligible.

That said, I prefer to do corner balancing in conjunction with the alignment and tend to rough balance, rough align, then fine balance and fine align.