Hi Taka
This is how I look at things:
"I have two daughters, who are muscians."
The phrase "who are musicians" is extra information (i.e. non-restricting). Thus, first you have a simple statement of fact:
I have two daughters.
Then you basically have an afterthought which amounts to something like this:
Oh, and by the way, both of my daughters are musicians.
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"I have two daughters who are muscians."
This sentence modifies the meaning of "two daughters", but it does not state that there are only two daughters. It only restricts the meaning of the particular two daughters being referred to. For example:
I have six kids. I have two daughters who are musicians. I have one daughter who is an ESL teacher. And all three of my sons are graphic designers.