You're not doing anything wrong. Combining with Simple Delivery is a minefield and often just doesn't work. When items are bought in one transaction at checkout you should receive a single postage label for them to be combined but obviously that isn't happening.
Ideally cancel/refund the transactions with buyer's agreement and relist in one listing with one postage for the buyer to buy again. You do that from the dropdown menu for the items on your orders list.
The alternative would be to send them together anyway. Open the unused labels to mark the items as dispatched then add the tracking number to the extra item at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser but if you can't manage it the payment will be held for 14 days.
If the buyer paid for the unused labels the cost will be automatically refunded to them in 30 days. You could put a note on your descriptions that any buyers wanting more than one item should contact you first so you can relist into one listing before they buy.
