It is currently Not possible to combine postage when using Simple Delivery. eBay say it's a technical issue that they are working on, but it's certainly been a while and no fix date being announced.
The best way here would be to cancel both items saying buyer requests to cancel , this will refund the buyer and your seller performance is not affected , then relist both items together, so you'll have one postage label and one tracking number.
If the buyer paid for the original labels they will be refunded for this.
You'd need to message the buyer to explain the actions taken above, and make it very, very clear in your messages that the items being delisted will be relisted for the buyer to buy on eBay again . Make it clear this is Not an off eBay sale, as eBay do read all messages going through their system.
@tr-194829