Sometimes writers omit a word in a compound structure. This type of structure, known as an elliptical structure, is appropriate when the word omitted later in the compound is exactly the same as the word earlier in the compound.

That bell belonged to the figure of Miss Duling as though it grew directly out of her right arm, as wings grew out of an angel or a tail [grew] out of the devil.

– Eudora Welty, One Writer’s Beginnings

If the omitted word does not match a word in the other part of the compound, readers might be confused, so the omission is inappropriate in formal writing.

The verb is does not match the verb in the other part of the compound (are), so the writer needs to include it.