Originally Posted by
FlyinHawaiian
If they are "Type B" (most common for this sort of compensation) they are normally not combinable, unless they are from the same booklet.
Type B vouchers normally are combinable unless they say otherwise, I believe the only non-combinable (or more accurately conditionally combinable) Type B vouchers are bump vouchers, and these don't sound like bump vouchers.
However, from the OP's description, I would guess these are Type A (Customer Service). Those are not combinable, though they can be used in conjunction with Type B vouchers.