At least it was a shared sentiment and intent between those two that they are only acting, to basically fool the parents, until they later at some point terminate the engagement. If one of them was serious, one was just acting, it would be emotionally pretty harsh. Naturally we can assume they are already falling for each other, but it's difficult for them to suddenly get serious, both believing the other is still only waiting for the whole thing to end. It would be like a breach of contract for either to suddenly ask if they perhaps couldn't start dating for real. No matter how subtly you presented it, the intent would be clear, after all. A difficult situation.
A third party getting involved would be the easiest situation. If both Arisa and Yuzuru told their genuine feelings to that third party, the third party could then inform both of them of it. After all, then there would be no danger of anything adverse happening.