Getting WhatsApp to work for both networks on a dual-SIM phone
You can install one app per phone So you cannot use it for more than 1 network in the same phone(dual sim phone)

From Whatsapp FAQ


  Your WhatsApp account can only be verified with one number, on one
  device. If you have a dual SIM phone, please note that you still must
  choose one number to verify with WhatsApp. There is no option to have
  a WhatsApp account with two phone numbers. If you attempt to
  frequently switch your WhatsApp account between different devices, at
  a certain point, you may be blocked from re-verifying your account. So
  please do not repeatedly switch between different devices.