If Eri confesses to Harima First, before Harima confesses to Tenma, wouldn't there be some kind of time lag where Tenma, may or may not notice that Eri is no longer herself and worry about her and possibly spend most of her time comfiting Eri with the rest of her friends? She could possibly find out who Eri Likes (With comfirmation from Eri) and Tenma would possibly start to avoid Harima, decreasing his chances to find a good time to confess to tenma.
But Since We know how School Rumble is... Harima will try deperately find a way to confess to tenma before tenma confesses to karasuma... and accidently confesses Yakumo this time LMAO! or accidently confess Eri again....either, or is good lol be funny if he confessed to both of them at the same time.

