If you'd like to combine all of the data from two accounts under one email address, you can use the "Merge accounts" feature, listed among the "Advanced features" on the account settings page of our website. Merging accounts this way is a permanent action – it will close one of your accounts, and move all of the groups, friends, and expenses from that account into your new "primary" account.
You can also merge group and friend invites into your existing account – just follow the invite link you are sent, then click the link that says "Already have an account? Click here!". Splitwise will then add the relevant group or friend to your primary account.
Both emails will still be associated with the merged account. To manage multiple emails, go to your account settings and hit "edit" next to your primary email address.