Sina Maria at Juan ang gumamit ng numero ng kanta.

Sina is used with two or more personal names.
Si is singular, so you use it for one person only, such as si Maria.

So:

• si Maria = Maria
• sina Maria at Juan = Maria and Juan

You can think of sina as the personal-name version of a plural subject/topic marker.

In this sentence, ang introduces the phrase gumamit ng numero ng kanta, which describes Maria and Juan.

The pattern X ang Y often works like:

• X is the one who Y
• It is X who Y

So:

• Sina Maria at Juan ang gumamit ng numero ng kanta
  = Maria and Juan are the ones who used the song number

Here, ang is not marking Maria and Juan. That part is already marked by sina.
Instead, ang introduces the verbal phrase that identifies or describes them.

Gumamit comes from the root gamit, which is related to use.

The verb uses the infix -um-, giving:

• gamit → gumamit

In this sentence, gumamit is an actor-focus form. That means the sentence is built around who did the action.

Since Maria and Juan are the ones doing the using, gumamit is the natural form here.

That ng marks the object of the verb gumamit.

So in:

• gumamit ng numero

the meaning is:

• used a number / used the number

With actor-focus verbs like gumamit, the thing affected by the action is often marked with ng.

The second ng links two nouns together. Here it means something like of in English.

So:

• numero ng kanta = number of the song / song number

This is a very common Filipino pattern:

• pangalan ng bata = name of the child
• kulay ng kotse = color of the car
• numero ng kanta = number of the song

So the two ng words in the sentence do different jobs:

• the first ng marks the object of the verb
• the second ng links numero and kanta

Because sina already covers the whole coordinated name phrase:

• sina Maria at Juan

You do not need to repeat it before each name.

So this is natural:

• sina Maria at Juan

Rather than repeating markers, Filipino often lets one marker apply to the full group.

Yes.

These two are close in meaning, but the given sentence puts more focus on Maria and Juan:

• Sina Maria at Juan ang gumamit ng numero ng kanta
  = It was Maria and Juan who used the song number

A more neutral version is:

• Gumamit ng numero ng kanta sina Maria at Juan
  = Maria and Juan used the song number

So the original sentence highlights who did the action.

Not very naturally.

A very rough breakdown is:

• sina Maria at Juan = Maria and Juan
• ang gumamit = the ones who used
• ng numero ng kanta = the song number / number of the song

But if you translate it word for word, the English may sound awkward. A more natural English rendering is usually:

• Maria and Juan were the ones who used the song number
• or It was Maria and Juan who used the song number

So this is a good example where structure matters more than literal word-for-word translation.

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

• Sina Maria at Juan ay gumamit ng numero ng kanta
  is a more straightforward topic-comment sentence.

• Sina Maria at Juan ang gumamit ng numero ng kanta
  is more like an identifying or emphatic sentence:
  Maria and Juan are the ones who used the song number

So the ang version is especially useful when you want to highlight who exactly did the action.

Yes. You would normally change the verb form and sentence structure.

For example:

• Ang numero ng kanta ang ginamit nina Maria at Juan.

This shifts the focus to the song number rather than Maria and Juan.

Compare:

• Sina Maria at Juan ang gumamit ng numero ng kanta
  → focus on Maria and Juan

• Ang numero ng kanta ang ginamit nina Maria at Juan
  → focus on the song number

This is an important feature of Filipino: the verb form often changes depending on what the sentence focuses on.