Metheus puzzle question

[Mooagain]

I finally found a friend to do the Metheus puzzle with, but we're stuck on the second part. We haven't been able to figure out how to check the code after putting it in. The wiki says we need to press it in unison, but what does that mean? Do we press at the exact same time, or does it trigger after we've both pressed it once? Is there any visual clue as to if the check has even gone through and had no results?

[Pruinae]

It doesn't need to be at the EXACT same time, but you both need to press it within the same 5~ seconds at least. Then, wait for it to update. There isn't really that clear of a visual clue but after some tries you get the hang of it. 

Also, don't retaliate the button, every press is pretty much one try lost. It is sensitive (｡•́︿•̀｡)