That is correct, for a Private seller when a case is opened only a full refund can be made.
This is the worse case scenario that can happen and has here, that is, posting 2 items purchased separately together, you only have one tracking, and therefore cannot prove the other did not arrive.
You are left with fully refunding.
If eBay step in and refund, should the case be escalated, if you think the decision for the refund unfair, you can appeal, link below shows the process.
I'll admit it's not easy, eBay do not see what was and what wasn't received, and they do tend to side with a buyer's view.
If you need to appeal this explains how:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@andhow23