You should both complete the process of opening your N26 bank account first. Once this is done, head to the Finances tab and select ‘Joint account’. You’ll be asked who you’d like to share your account with. Choose the person from your contact list, answer a few questions on your financial background, relationship, and usage, and accept the terms and conditions to send the invite. After they accept your invite they'll be able to access the joint account.
If you’re unable to find the other person in your contacts and are both using N26 head to your app settings. There you’ll go to Personal details > personal information and toggle on "allow other N26 users in my contact list to find me for easier transfers".