I don't know about UA but on AA you can combine them but you don't really want to! WHY? because each voucher allows the US taxes to be waived on all tickets purchased, so if you use a voucher smaller than the price, you pay the difference, but the next voucher does the same on the next ticket. On a trip to Antigua one spring break a couple of years ago, the three of us totaled $5000 in vouchers with 2 bumps (on AA). I rotated the vouchers but didn't realize to spread them out. Now I am. With the last $99 voucher balance from that windfall, I used it towards 2 tickets to London and the US taxes on both tickets was eliminated. So rotating you vouchers is the best strategy.

------------------
DtG