Martha has two sons, who are still at school, and two, who are at university

Here's a quote from A Student's Introduction to English Grammar:

[11]   i  Martha has [two sons who are still at school] and [two who are at university].  

In [i] the relative clauses certainly are semantically restrictive: they distinguish two sets of sons (evidently Martha has at least four in all).

Is it possible to add commas like this?

Martha has two sons, who are still at school, and two, who are at university.

If so, does this mean that she has only four sons, not that she has at least four?

With the commas in your last example the sentence could be reduced to

Martha has two sons, and two.

This does not make sense. To say that she has two sons at school and two sons at university you would need

Martha has two sons at school, and two at university.

The comma here could be left out as well.

The sentence does not strictly imply that Martha has no other children, but it is reasonable for a reader to expect that there are no others.