I due avversari si alleano.

Yes. I is the masculine plural definite article, meaning the.

Here it agrees with avversari, which is masculine plural:

• il avversario is not correct because before most words starting with a vowel-like sound? Wait—here the singular is l’avversario
• plural: gli avversari

So you might expect gli in the plural, but after due, Italian commonly uses:

• i due avversari = the two opponents

This is a very common pattern:

• i due amici
• i tre ragazzi
• le due case

So i here is the article for the whole noun phrase the two opponents.

Both are possible, but they mean slightly different things.

• i due avversari = the two opponents
  This refers to two specific opponents already known from context.
• due avversari = two opponents
  This is more indefinite.

So the article i makes the phrase definite.

Because in Italian, numbers normally come before the noun:

• due avversari = two opponents
• tre libri = three books
• quattro giorni = four days

That is the standard word order.

Avversari is the plural of avversario, which means opponent, adversary, or sometimes rival, depending on context.

Forms:

• singular: l’avversario
• plural: gli avversari

It is a masculine noun.

Yes, si is part of the pronominal/reflexive-style verb allearsi, which means to form an alliance, to ally oneself, or more naturally in English, to ally with each other / to join forces.

In this sentence:

• si alleano

This does not usually mean each one is acting on himself individually. Here it has a reciprocal sense:

• the two opponents ally with each other

This is common in Italian. A plural subject + si often expresses a mutual action:

• si vedono = they see each other
• si aiutano = they help each other
• si alleano = they ally with each other

Alleano is present indicative, third person plural of allearsi.

So:

• io mi alleo = I ally myself / I form an alliance
• tu ti allei
• lui/lei si allea
• noi ci alleiamo
• voi vi alleate
• loro si alleano = they ally themselves / they form an alliance

In your sentence, the subject is plural (i due avversari), so the verb is plural too: si alleano.

Because alleare / allearsi is a first-conjugation verb in -are, and in the present tense the third person plural ending is -ano.

Pattern for many -are verbs:

• parla = he/she speaks
• parlano = they speak

Likewise:

• si allea = he/she allies himself/herself
• si alleano = they ally themselves / they ally with each other

Not exactly.

• allearsi is the usual verb for to ally oneself, to enter into an alliance, to join forces
• alleare exists, but it is less common in everyday use and can sound more formal or technical depending on context

In normal Italian, when talking about people, groups, or countries forming an alliance, allearsi is the natural choice.

Examples:

• I due paesi si alleano.
• Le fazioni si alleano contro il nemico.

Yes, in a grammatical sense the Italian present can cover several English present-time ideas depending on context:

• The two opponents ally with each other
• The two opponents are forming an alliance
• The two opponents are joining forces

Italian present tense is often broader than English present simple.

Yes, but the basic order here is the most neutral:

• I due avversari si alleano.

Other orders are possible for emphasis or style, for example:

• Si alleano i due avversari.
• I due avversari alleano is not correct here, because you need si

Italian allows some flexibility, but the standard learner-friendly version is the one you have.

Good question. On its own, the plural article is indeed:

• gli avversari

But when a numeral like due comes before the noun, Italian commonly uses the regular plural article that matches the noun phrase as a whole:

• i due avversari
• i tre studenti
• i due amici

This is standard Italian. So even though you say gli avversari, you say i due avversari.

A helpful pronunciation guide is:

• si = see
• alleano ≈ al-LEH-ah-no

A rough full pronunciation:

• ee DOO-eh av-ver-SAH-ree see al-LEH-ah-no

A few notes:

• due has two clear vowels: du-e
• avversari has stress on -sa-
• alleano has stress on le

Usually, yes.

• avversari = opponents, adversaries, often in competition, politics, debate, sport, or conflict
• nemici = enemies, usually stronger and more hostile

So i due avversari si alleano suggests that two opposing sides, rivals, or competitors decide to cooperate. It does not necessarily mean they were bitter enemies.