Q:

Does mixing alcoholic drinks really make you more drunk?

There is plenty of anecdotal evidence ("beer after wine and you'll feel fine, wine after beer will make you feel queer") that mixing alcoholic drink types leads to a stronger effect, but I can't find any true studies. 
In fact the only studies I found are looking at mixing energy drinks and alcohol (mixing with energy drinks increases motivation for more alcohol (in college students) [1]), and discussed mixing caffeinated beverages with alcoholic ones [2].
Are there any studies specifically looking at mixing alcoholic drinks? They would have to compare people drinking the same amount of alcohol, but some people mixing, others drinking the same thing. Maybe even a cross-over study design? Same people do both, one after the other? The only way to get an unclouded answer!
I am also interested in the follow-up why question: Why does mixing some drinks make you more drunk? Presumably it is something in wine (for example) that interacts badly with something in beer at the chemical level (the metabolites maybe)?

Marczinski CA, et al, (2012). Mixing an Energy Drink with an Alcoholic Beverage Increases Motivation for More Alcohol in College Students. Alcohol Clin Exp Res, epub. doi: 10.1111/j.1530-0277.2012.01868.x
Touyz LZ, (2011). Mixing drinks and concocting troubles. Curr Oncology, 18(6):262-3. PubMed; FullText.

A:

There are some very general answers to your question.

Definitely, there is a lot of magical thinking. We as humans are very prone to anecdotical evidence and extrapolations from incomplete data, even more so when we are drunk.
As an interesting "proof" is the fact that the German counterpart of the saying "beer after wine and you'll feel fine, wine after beer will make you feel queer" is "Wein nach Bier, gönn' ich Dir, Bier nach Wein, lass es sein" -- which means the exactly opposite of the English saying (beer after wine is bad, wine after beer is fine).

Many kinds of alcohol contain unwelcome additional substances, for example fusel alcohols. Definitely, "mixing" broadens the range of these additions.

Mixing can lead to a misperception about the actual amount of alcohol that was taken in. Many people who would not drink a vodka and drive have no problem driving after a half a liter of beer (which contains same amount of alcohol two shots of vodka).

