idea about antibiotics

[Olli Niemitalo]

I'm a complete amateur in this field, but got an idea i find 
"brilliant" (huh?)...

Why are antibiotics given only one sort at a time? If you gave say two
different sorts of antibiotics at a time, the probablility for a microbe
to develop resistence would be in my thoughs be nearly to 2nd power 
-> extremely small! Why did they never think about this, or am i being 
the stupid here?

-olli