Whatsapp on 2 devices: "does not work, tell why!"

After nearly everyone of my friends is using whatsapp, it was overdue to download this app.
Why is this app used by everyone? What's cool about this app?

• You have to do nothing after the download except this sms-verification
• No account creation
• No new password (or reuse another one <- li="" nooo="">
• No need for searching addresses
• You do not have to take care about your contact list
  
• Just start messaging

Why does this work? Whatsapp uses your phonenumber to build the account identifier. The app scans your phone address book and builds its own contact list. Fine.

Really?

Not at all: If you want to add whatsapp on a second device with no sim card (e.g. your tablet), you can find several instructions but: after adding whatsapp to this device, whatsapp on your phone does no longer work...
Whatapps says:

Your WhatsApp account can only be verified with one number, on one device.
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.
Cheers,
WhatsApp Support Team

Finally i would say: The biggest advantage is also the biggest disadvantage. Using the phonenumber as account id makes everything easy but prevents the usage of more than one device....