How to upload 2 tax returns?

Sorry---you cannot transfer two tax files into a new return.   Since you are starting a new joint return, transfer the most complicated one and enter the information for the other spouse yourself.   When you begin a new joint return you are asked in My Info  if you were married in 2021.  Say yes.   Next question will be if you want to file together with your spouse.   Answer yes to that.   Then you enter the information for the other spouse.   As you go through the various income screens, you will see spots with each spouse's names.  Take care to enter the income under the correct spouse's name.

Best Wishes!

@aklinslow  You didn't ask, but don't try to change the order of the names on the joint return. Whoever's return you transfer will be listed first on the joint return. It's nearly impossible to change the order of the names without causing errors that you won't be able to fix.

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 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.

You should usually  file a Joint return which is only 1 return combined for the both of you.  Make sure to stick with this one account for the future.  Just ignore the other spouse's account.

Getting Married FAQ

https://ttlc.intuit.com/community/tax-filing-status/help/what-does-getting-married-mean-for-my-taxes/00/1756642

And info on Getting Married…..

http://turbotax.intuit.com/tax-tools/tax-tips/Family/Getting-Married/INF12006.html