Unfortunately, you cannot now add tracking for the second item and that payout will be held for a minimum of 14 days.
If the buyer has paid for the postage, worth sending them a message explain both items are being sent in one parcel, and they will be refunded for the unused postage, this can take 30 days.
There is currently a long, long standing technical issue with the inability to combine postage, when using Simple Delivery.
@andscof44