Just tried again and can report my highest score is 24 on the left arrow and 34 on the right. I really don't think I can get any faster! Can you do it yourself? I'm worried I might be doing something wrong - just bash both arrows as fast as possible, yes?
Yes, you're doing it correctly. Try to press right and left at same time, it should register both entries (at least I tried it in two different machines). But I changed both values to 25, I hope you can win it now :)
(Yes, I can reach 40, but just after 2 or 3 tries xD)