Why are you not allowed to use 2 license/authentication codes on the same device? I.e bought two TurboTax softwares and want to use them both on the same computer.

The license code is not the problem. You cannot have two copies of personal TurboTax for the same year installed on one computer.

Why do you want to have two copies of the TurboTax software?

You don't need 2 programs or licenses.  What versions did you buy?  All the Desktop programs have the same forms. You just get more help and guidance in the higher versions.  So one program can handle all your returns.  One Desktop program can do unlimited returns and efile 5.  

Do you need to efile more than 5?   Sorry you can not buy more efiles, you can not efile more than 5.  It is an IRS limit.  If you need to efile more you have to buy Intuit Products for Tax Professionals https://taxpro.intuit.com  and sign up with the IRS as an ERO - an Electronic Returns Originator.  How to become an Authorized E-file Provider…..

http://www.irs.gov/Tax-Professionals/e-File-Providers-&-Partners/Become-an-Authorized-e-file-Provider

Maybe some returns can use the Online version?