You can use part of it on one purchase, and have the remaining balance for another purchase.
However, you can't choose how much of each. The system will use as much of the voucher as it can before applying remaining balance to a credit card.
Also, vouchers can not be used to pay taxes and fees, so you will need to use CC, gift card, or travel funds to cover those.

Finally, vouchers can be used to purchase a ticket for anyone, not only for yourself.
If you are booking for more than one person, you might use up the whole amount across the different passengers.