You'll have to read the competition rules. Some allow you to enter two in the same category if they are in different subcategories. For example, American IPA and English IPA, 14B and 14A. Some only allow one IPA (for example) to be entered.
I don't know about entering two in the exact same subcategory. I haven't seen one that does allow it (but I'm sure there might be some).
Entering the same beer into two different categories is completely fine. If you have an American Pale Ale but don't know if it is in IPA territory, you can enter it into 10A and 14B to see where it is sitting. Remember though, at a certain point the whole thing becomes subjective...