Hi, i dont know which phone you are using but if you already have configurated the device which sim should be active for the data plan, then it is the only matter of which phone number you tell whatsapp at the first time after the new installation.
Whatsapp only checks one time if the phone number belongs to the rightful owner and even that you dont need a dual sim phone, because you can receive the code registration sms on the one phone and insert it on the other one manually.
Sent from my GT-S7562 using xda app-developers app