Unfortunately, you can’t. You will only be able to use one of the myDNA apps at a time. You can move between the Unlocked and Pregnancy app at any time, however, be aware that you will lose any saved shopping lists or meal/workout programs when you switch from one app to the other.
In order to switch between apps, please get in touch with our customer support team to enable your account for both applications. Alternatively you could just register for the second app with a new email address.