Is it acceptable to use an exclamation point after a question mark to emphasize the question?
@Csomers, this actually depends more on the context of the writing than anything else. For a more formal writing context (e.g., academic articles), combining punctuation (and, sadly, rare punctuation like the interrobang, ‽) are often frowned upon.
On the other hand, if the question is being asked in quotes—that is, someone like a character is actually speaking it—then there may be a strong case for using it. Or, if the context of the writing is less formal (e.g., in communications with friends), such symbols are often considered acceptable.
Personally, I feel that these marks convey a perfectly valid idea (the combination of interrogation and exclamation). And though I do so love the interrobang, I can imagine times when I would prefer to stress which emotion is more important in the exchange (which would lead me to use both separate punctuation marks and place the more important mark first).
If it matters at all, the Unicode Consortium believed that the use of such combining punctuation marks is valid enough that they should receive their own code points (
U+203C - ‼,
U+2047 - ⁇,
U+2048 - ⁈ and
U+2049 - ⁉).
Quite aside from the question of the social or situational acceptability of adding an exclamation point after a question mark "to emphasize the question" (which as HalosGhost notes depends to a great extent on the intended audience), I think you need to pay attention to the effect that introducing the extra punctuation may have on the meaning of the question you're asking.
The most significant difference between "What are you doing?" and "What are you doing?!" it seems to me, isn't that the latter question has greater emphasis than the former; it's that the force of the added emphasis appears to express outrage or incredulity at what the addressed person is doing, whereas the standard question-mark punctuation indicates a simple desire to know what the addressed person is doing.
In the case of "What are you doing?!" or "Is that what you call fair?!" the added exclamation point also has much the same effect as italicizing (or underscoring the final word of the question. This can be the selective emphasis you intend, or it can be at cross purposes to that intention. My advice is, Proceed with caution.