Sounds like you purchased a flight using one of the two vouchers and paid the remaining balance in cash? Was it a Wanna Get Away fare/ticket?
If that's the case, yes you could cancel the ticket you purchased BUT the amount you paid (including the cash amount) would be returned to you in one Travel Fund expiring when the voucher was scheduled to expire. You would then have a Travel Fund for the whole amount of your trip AND the second voucher so you wouldn't achieve what you want to do, assuming that is to use both vouchers and not spend your cash. At this point I don't see how you could use both vouchers and get your cash back.
However, if you bought an Anytime or Business Select fare you could get the cash portion back if you canceled and then you could rebook.
