Can I use the same account from multiple devices?

1. If you register your phone number (i.e. generate a password) using whosthere, your previous password will get invalid. Thus WhatsApp will stop to log in on any other device. You will be able to re-register with the official client, but that will stop whosthere from logging in.

2. You can share your password between different instances of whosthere (without re-registering), but you can only have one connection at a time. If you open a new connection, the old connection will be closed by the server.

Thus it's best to use a seperate phone number for each client. Phone numbers can be obtained from services such as skype, google voice, etc.