You can't do anything to show the untracked item has been delivered so payment will be held for 14 days but you will get it.
If you want to mark the second item as dispatched you can do that by opening the unused label (Get prepaid label > Get prepaid label and QR code). It will remain unused and if the buyer paid for it they will be refunded automatically in 30 days. You don't need to refund any postage.
For future reference an alternative would be to cancel/refund the transactions with the buyer's agreement and relist with items on one listing and a single postage cost for the buyer to buy and pay again. Also buyers can get combined postage by putting each item in their basket before they check out. (on a PC, not the app)