I also have two holes in my skimmer... not clear about why that is, but i saw a diagram of one of these & it looks like they connect to one pipe about a foot down anyway...
btw, I find that annoying because it means that a vacuum filter basket ( small basket with hole cut through to allow KK to fit through & still skim) , won't fit because the hole that you put the KK into is too close to the back wall of skimmer.

As for main drain openings... besides actually swimming to the botom of the pool & checking for suction,
go to your pump area & see if Two pipes become one as it enters the pump.
One pipe should be the skimmer pipe & the other should be the main drain.
check to see if there are some type of gate valves here... if there are two valves, are they both pointed in the same direction?

hope this helps, I'm a newbie here & that was my best guess