As my spoiler-filled comment elsewhere in this thread makes clear, I agree that CoR isn't as good of a book as CoT. However, I still think it's a very good book. And, more importantly to your point, I don't think it detracts from CoT in any way.

I know what you mean where sometimes a sequel can ruin (pun!) one's feelings about the first book in a series. This sometimes happens because a sequel is so awful that it leaves a bad taste about the entire series, and it sometimes happens because new information is revealed that retrospectively messes up plot points one liked in the first book. I don't think either of those things are true about CoR.

So even if you don't like it as much as CoT, I don't think it will impact how much you liked CoT. In fact, I think it's still additive even if it's not quite as great.