Два события произошли одновременно.

Because событие is a neuter noun.

In Russian, 2 has two forms:

• два for masculine and neuter
• две for feminine

Examples:

• два стола = two tables (стол is masculine)
• два окна = two windows (окно is neuter)
• две книги = two books (книга is feminine)

Since событие is neuter, the correct form is два события.

This is a very common point of confusion.

After the numerals 2, 3, 4, Russian usually uses the noun in the genitive singular form:

• два события
• три события
• четыре события

But after 5 and higher, it normally uses the genitive plural:

• пять событий
• шесть событий

So:

• событие = event
• события = can mean either
  
  • events in some contexts, or
  • the form used after 2, 3, 4
• событий = genitive plural, used after 5+ and in other contexts

This is one of those places where Russian form and English logic do not match neatly.

Because the meaning is still two events, so the verb agrees with the real number of things involved.

So Russian says:

• Два события произошли = Two events happened

The verb произошли is the plural past tense form.

Compare:

• Одно событие произошло = One event happened
• Два события произошли = Two events happened

Even though the noun after два takes a special form, the subject is still plural in meaning, so the verb is plural too.

The dictionary form is произойти.

Here is the breakdown:

• произойти = to happen, to occur
• произошли = happened / occurred (past tense, plural)

This verb is perfective, which means it presents the event as a completed whole.

So произошли means that the events did happen as completed events.

This is about aspect, one of the most important parts of Russian verbs.

• произойти / произошли = perfective
• происходить / происходили = imperfective

In this sentence, the speaker is simply stating the fact that the two events happened. Russian often prefers the perfective for that.

• Два события произошли одновременно.
  = Two events happened simultaneously.

If you used происходили, it would sound more like you are focusing on the process or duration:

• Два события происходили одновременно.
  = Two events were happening at the same time.

That version is possible, but it has a slightly different feel.

Одновременно is an adverb.

It means simultaneously, at the same time.

Because it is an adverb:

• it does not change form
• it does not have gender, number, or case
• it modifies the verb phrase, telling us how/when the events happened

So in:

• Два события произошли одновременно

the word одновременно tells us that the two events happened at the same time.

Yes. Russian word order is more flexible than English word order.

The neutral version here is:

• Два события произошли одновременно.

But other orders are possible, for example:

• Одновременно произошли два события.
• Произошли одновременно два события.

These all mean roughly the same thing, but the emphasis changes.

For example:

• Одновременно произошли два события.
  puts more attention on simultaneously
• Два события произошли одновременно.
  is a straightforward, neutral statement

So the original sentence is natural and neutral, but it is not the only possible order.

Yes, in many contexts you could say:

• Два события случились одновременно.

That is understandable and grammatical.

But произошли often sounds a bit more neutral or formal when talking about events, especially in writing, news, history, or analysis.

A rough comparison:

• случиться = to happen
• произойти = to occur / happen

Both are common, but произойти fits very naturally with событие.

In fact, событие произошло is a very standard combination in Russian.

A helpful stress guide is:

• два́
• собы́тия
• произошли́
• одновре́менно

So the sentence is pronounced roughly as:

два́ собы́тия произошли́ одновре́менно

A few notes:

• событие has stress on бы́
• произошли has stress on the final -ли́
• одновременно has stress on вре́

Stress matters in Russian, so it is worth learning these together with the words.

Because Russian has no articles like English a/an/the.

So:

• Два события произошли одновременно.

can mean:

• Two events happened simultaneously
• The two events happened simultaneously

The exact interpretation depends on context.

Russian speakers do not need an article to make this sentence complete and natural.

Yes, it is common, but it is a bit more formal or neutral than some other words.

• событие = event, occurrence

It is very common in:

• news
• history
• formal writing
• discussion of important happenings

Examples:

• Это важное событие. = This is an important event.
• Какие события произошли вчера? = What events happened yesterday?

So in this sentence, события is a very natural choice.