Because of the logical relationship between the first and second functions, it simply doesn't make sense to have two extraverted functions. It would be unclear what the motivation of such a person would be. The information element corresponding to the first function is perceived as being objective and fundamental, while the second information element is perceived as being changeable and submitting to the logic of the first.
For example,

. Reduced "formula" (from
http://www.socionics.us/philosophy/formulas.shtml):
"Works with objects' emotional activity and expression to achieve harmony and preserve integrity in external qualities of objects and situations."
This doesn't make sense. How does stimulating people's emotional life relate to preserving the integrity of their external qualities?
Now take

for contrast. This means we have a person who perceives objects' external qualities as being objective and fundamental and changes the subjective relationships between objects to reflect and match these qualities.
That makes more sense.