can I file 2 tax returns on premium under 1 account? 1 for me and 1 for my spouse

Why do you  need 2 tax returns?  Aren't you filing a Joint return?   Online is only good for 1 return per account so you would need 2 accounts.  If you do a second return in the same account it will overwrite the first one.  If you need to do more than 1 return  you should consider using the Desktop download program.  It can do multiple returns and efile 5 federal for free.  And you can  use a lower version since all the Desktop programs have the same forms.  

You can start another return in TurboTax Online by:
Logging out if you are in your account
Choose what TurboTax Online product you want to use for the second return
Create your account screen
Set up a new login for the second return
Start working on the new return