Learn Italian: Quei due non fanno altro che bisticciare.

This is a set structure used to express an exclusive action, meaning to do nothing but or to do nothing other than. You conjugate fare to match the subject (here, fanno for quei due) and follow it with altro che plus the infinitive of the action verb, which is bisticciare.

Why is it Quei due and not Quelli due?

When a demonstrative adjective precedes a number like due, it follows the same rules as if it were placed before a regular masculine noun starting with a consonant. Therefore, quello becomes quei. We only use the pronoun form quelli if it stands completely alone without a noun or number immediately following it.

Could I replace che with di before the infinitive bisticciare?

No, in this specific construction you must always use che to introduce the infinitive. While di is often used to link conjugated verbs to infinitives in other Italian structures, altro che operates as a fixed comparative phrase here meaning other than or but.

What is the difference between bisticciare and litigare?