Yes, assuming this is a TCVA-type voucher you can. If you have, say, a $500 and want to use on 2 - $200 tickets and half of a third, you can do this. These kind of vouchers work almost like a gift card - you can use them again and again until the balance is gone.

If their are multiple passengers on one itinerary, you should be able to enter in the Promo code/PIN as a form of payment and apply it to the first person on the reservation (first), then re-enter the promo code for person 2, etc. until it is used.

Alternatively, if you use it on a single-person itinerary for yourself, the amount will be removed from your balance, and the remainder will be available for use on future itineraries up until the expiry date or all is used.