It was great having WhatsApp on the iPad but due to this lack on synchronisation I had to make the decision to remove it from the iPad.
What I found happened when having this dual-install is whichever device was used last to send a message would receive the reply, the other device would not even know that a message was sent or received. This is obviously not ideal.
I'm guessing this is due to the XMPP being used for messaging by WhatsApp and keeping a connection to a single device only. That makes sense but it would be nice if it made use of cloud storage for example so that all devices get all the updates. Just my two cents.