Learn Arabic (Modern Standard): القميصان نظيفان.

The suffix ـان (-aan) is used for the dual form when the words are in the nominative case (the subject or the main information of a sentence). Because القميصان is the subject and نظيفان is the predicate (the word giving information about the subject), they both take the nominative ـان ending. If they were the object of an action or followed a preposition, they would end in ـين (-ayn) instead.

Why does the adjective نظيفان have a dual ending too? In English we just say "clean", not "two cleans".

In Arabic, adjectives must match the noun they describe in number (singular, dual, or plural) and gender. Because القميصان (the two shirts) is a masculine dual noun, the adjective describing it, نظيفان (clean), must also take the masculine dual suffix ـان.

Why does the first word have الـ (al-) but the second word doesn't?

This is how Arabic builds a basic "A is B" sentence without using a word for "is" or "are". The subject القميصان (the two shirts) is definite, so it has الـ (the). The predicate نظيفان (clean) is indefinite, leaving off the الـ. This difference is what tells you it's a complete sentence meaning "The two shirts are clean." If both had الـ, it would just be a descriptive phrase meaning "the two clean shirts".

Is there a short vowel at the very end of these words?