Combining postage is currently not possible it's a technical fault eBay say they are ' looking into to fix'.
If the buyer has paid for 2 postage costs and not asking for the items to be combined, best really to send them separately, then you have those 2 all important tracking numbers.
Or, you can cancel the orders, this refunds the buyer and relist them as one listing. You'll then have one postage charge for the buyer to pay, and one important tracking number.
If you wish to do this, click on the item top left >> View order Details, from More Actions on the right >> Cancel this order, citing Buyer Requests to cancel, this refunds the buyer, and your seller performance is not affected. Do the same for each item.
You would have to message the buyer inform them what you are doing , as you do need them to agree. Be very clear in your messages, eBay do read them, that the items being delisted WILL BE relisted on eBay again, for the buyer to buy,. Make it very clear this is not an off eBay sale.
@americanpie52