So your wife’s had the notification that her card’s been dispatched & you’ve just received yours & a spare?
In that case she can use that card, Monzo cards aren’t linked to your account until you complete registration. Just swipe left with 3 fingers on the queuing screen to & then you’ll be able to get set up
Then check out this post with tips for new Monzo users
https://community.monzo.com/t/so-youve-just-signed-up-to-monzo-what-next-tips-bulb-open-wiki/6613/1