It used to be that the package was only for the first showing however, someone on the DIS recently posted that they were told it could be for either show. I've only heard this once, from one poster though.
If that is the case, I believe you would tell the waitress when you are seated and she will fill out the voucher accordingly.
I'm not 100% sure though that you can now use the voucher for both shows. Maybe you can call dining as ask them.
Good luck!