Never had one,
Can it be used to read and listen quick whatsapp and and send back msg?
Also can we be connected with 2 bloutouth device at same time, head phones/car and watch?
I have the galaxy gear s2 and it can be used to read whatsapp msgs as well as email, text, phone calls... Responding is a bit tricky. It has a keyboard but its not really efficient to type on it. I see the msg and decide if its important enough to pull out my phone to respond. My main use is that I can leave my phone sitting on a wireless charger on my desk and as long as I am within bluetooth range (IME about 40-50 feet) I can still see all my msgs. I also use it to answer phone calls when im holding things and cant hold a phone, although its on speaker so it only works if its not a private convo. I never would have bought mine but now that I got it I find myself using it a lot.
As for your 2nd question, your phone stays connected to your car and watch at the same time