No you cannot complete multiple tax returns using the TurboTax online editions. The online edition is only good for one tax return per account and User ID. To start another tax return using the online editions you would need to create a new account with a new User ID.
Up to 5 User ID's can be associated with one email address.
No you can't. The online product can only be used to prepare one tax return on each account. You can buy and download the CD/download product and install it on your computer to do the same thing you did when buying a disk from Target. You can complete an unlimited number of returns and efile up to 5 with one purchase.
Here is how to buy it from TurboTax: https://turbotax.intuit.com/personal-taxes/cd-download/
You can also look at buying and downloading it from other retailers where it may be less expensive. Some retailers sell versions without a state included so if you need to prepare a state return make sure it includes state.
If either or both of you qualify you could use one of the IRS FREE FILE OPTIONS : https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free