The Mythbusters had a recent episode testing if drinking beer and liquor would make you more sick thann drinking just beer. They got the myth wrong in the first place. The real myth is that if you drink beer before liquor you will get sick, but not if you drink liquor before beer.
Their test had Tory and Grant drink the exact same amount of alcohol whether they were drinking beer or mixing beer and liquor. It should be pretty obvious that consuming the exact same amount of alcohol should yeild the same results.
The reason the real myth holds is because, generally, when someone starts drinking beer, and then switches to liquor, they usually have consumed several beers and have a buzz, and then consume a significant amount of liquor. Liquor usually goes down quicker because it is in more concentrated quantities. When someone drinks beer and then liquor, they generally have a stronger buzz, and aren’t downing beers left and right. It doesn’t matter if you mix beer and liquor, and in what order you mix them in, if you drink too much alcohol, you will pay the price.