Etiquette for Splitting an Entree?

With the larger portions these days as well as the larger prices, I find that I'm splitting the entree with my S.O. more often. I realize that some restaurants have a split charge and that's fine. However, charge or no charge for splitting, do you adjust the tip based on the cost of 2 entrees? Or do you just tip normally based on the actual total? I have been making a small adjustment in favor of the waiter, but lately I am wondering if this is expected.

Also, does this logic apply if you just get appetizers?

I would say always tip according to the amount of the check, but I am sure the servers around here would be a better authority on the subject.

I tip on the bottom line (food and booze), doesn't matter what I order.

Side note: I was at a birthday party at Red Star Tavern a month ago with a large group and the tip was added to our check. I thought it seemed really low, so I ask our server what the deal was. He said that they don't/can't figure the tip on alcohol. We had about $35 worth of drinks on our ticket that the server or bartender were not going to be tipped for. We adjusted the tip, but found it strange that tips are not figured to include booze.

One thing that I'd like to point out. If I'm paying a split plate charge, then I fully expect the kitchen to put it onto two plates.
I personally don't care for the server just plopping down an empty plate in front of my wife and for me to have to fumble around with splitting it at the table.
It has happened to us at a couple of what I consider to be "better" but not "fine dining" establishments.
I generally tend to tip on the high side. But if I have to split my food at the table after I asked for it to be on two plates, you can bet that the server is only going to get 15% on the check total.
It might not been their fault; but I'm certain they could have said something in the kitchen for what I see as a resonable request that I'm paying a surcharge for.