It depends on what type of vouchers they are; this will be on the lower right of the coupon. If they are "Type B" (most common for this sort of compensation) they are normally not combinable, unless they are from the same booklet. Since they are in different names, this would also likely make them not combinable. You might want to try a search or look at the Forum FAQ for some of the other threads on this topic; again it really depends on what type of voucher (A, AB, or B) you are dealing with.