Hi you must connect your google account. You can see which account that has been selected by going to whatsapp settings --> chats --> chat backup --> google account. If you for some reason can't select your standard google account containing all your contact, please remove the twinapp and install it again. If it still refuses to let you choose your standard account in the first step, then skip this and go back to the chat backup afterwards to input your standard google account.
You will only be able to add new contacts with the primary whatsapp app but your twinapp will sync that contact after a while