A "family act" law (don't know exact date) permits muslim second marriage ONLY AFTER THE FIRST WIFE GIVES PERMISSION (this is to ensure both families are treated equally with 1st wife allowing it). But in reality a man (like Rashid ) might prefer one over other (which is NOT the muslim way). We don't know the circumstances under which Rashid married Shireen yet. I guess we will soon find out.
I am muslim and I have never seen or known anyone with 2 families at the same time... so might be very archaic/old system...not widely practiced anymore ( at least not in south east Asia). Usually people divorce and marries some one else. Even after divorce you are supposed to provide for the 1st wife and family (just like any other religion).