Hi adamvv, welcome to AndroidPIT! 
WhatsApp is always relying on the phone number, as it defines your account. When you connect the Web App to your phone app, it will stay connected. If you are using a dual sim smartphone with multiple instances of WhatsApp, you need to activate multiple instances of the Web App in your browser. Honestly, I am not even sure if you can actually do that within the same browser, if not, simply use a 2nd browser for the 2nd Web App.
— modified on Jun 13, 2018, 10:29:05 AM
Liebe Grüße, Bastian
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