Were the items purchased via the same transaction, or as two separate purchases? If it's the former then put both items in the same package, affix one of the labels to the package and post it back to the seller. Void the other label.
In the event that you bought the items via two separate purchases then you will need both postage labels, as each label will have a tracking number unique to the item in question that you are returning. If you put both items into one package and only use one of the labels then the seller could just tell eBay that he/she has received the first item, but claim that you still haven't returned the second item. As there would then be no tracking for the second item eBay wouldn't force the seller to refund you for that item, as they would probably just think "Well, if the buyer hasn't returned it yet, despite being issued an eBay Returns Label to post it back with, then we can't make the seller refund the money." That way you lose out on one of the refunds and the seller is only obliged to issue a refund for one item - ie: the one that the tracking proves has been received. The seller could then potentially relist the other item for sale on eBay again at some point, sell it to somebody else and end up receiving two separate lots of payment for the same item!
In summary if you purchased each item separately then return them to the seller separately, using both of the eBay Returns Labels provided. That way once the tracking proves that both items have been successfully returned to the seller you ought to get your money back for both items.
