With the online version, one account, one return. Period. If you do a 2nd return on the same account, then the first return will be over written and lost forever, with no possibility of recovery. But you can create up to 5 online accounts per e-mail address.
When using multiple accounts, it's important to use the same exact account each year for the person that a tax return was prepared on last year. Otherwise, you will be importing the WRONG information from the prior year's return.
