Learn Spanish (Spain): Las dos camisas son iguales.

The word igual means "identical" or matching in characteristics. If you said las mismas camisas, it would mean "the exact same shirts" (as in, the exact physical shirts we wore yesterday). Because we are comparing two separate shirts that look identical to each other, we use iguales.

Why is it son iguales using the verb ser instead of estar?

We use a form of ser (here, son) because being identical is seen as a core characteristic or defining physical trait of these shirts. The verb estar is generally used for temporary states or locations, not built-in descriptions.

How do we form the plural of the word igual?

Because the singular word igual ends in a consonant, you must add -es to make it plural, which gives us iguales. If an adjective ends in a vowel, you usually just add an -s.

Does las dos just mean "the two"?

It literally means "the two", but in Spanish, las dos (or los dos for masculine nouns) is the most common way to say "both". So you can also perfectly translate this sentence as "Both shirts are the same."