Did you both use Turbo Tax last year?
You can not transfer from or combine 2 returns or accounts. Just pick the return for the spouse that has the most complicated return to enter or has any depreciation or business or more investments. Or use the account/return for whoever you want to be listed first going forward (by the way you can not change the order of the names). Then add the other spouse's name, ssn and info to it.
But if you want to keep the prior accounts separate from the new Joint account then start a new account. Online is only good for one return per account. But you can use the same email address for 5 accounts.
Getting Married FAQ
https://ttlc.intuit.com/community/tax-filing-status/help/what-does-getting-married-mean-for-my-taxes...
Tax info on getting married
https://turbotax.intuit.com/tax-tips/marriage/getting-married/L0DvEUlEC