That is correct under Simple Delivery it is not currently possible to combine postage.
If the buyer has paid for 2 postages then you should get another label for the second item and send them separately, then you will have 2 tracking numbers.
Sending them together with one label and one tracking you will not be able to mark the second item as dispatched, and trust the buyer on receipt would not claim only one item is received.
Send them separately with 2 labels so you have those all important, 2 tracking numbers.
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