It sounds like the buyer bought the items separately, rather than asking for a combined total?

'If you're a private seller, items listed with Simple Delivery will automatically be combined at checkout. If a buyer completes checkout in separate transactions, the postage won’t be combined. To ensure you're protected, you should send items that aren’t combined using the Simple Delivery label provided.'

So, apart from cancelling & relisting so you can post both items together, yes, I think you'll have to post them both individually.

Unless it's possible for you to pack both together, and update the tracking so it registers that both items have been shipped? Not sure if that would work, as both items would end up with the same tracking number...?!

Posting has become a nightmare... 😱

(Or perhaps they couldn't be combined because one was Evri, & the other was RM?)