Yes. You can do unlimited returns in the Desktop program. You can't combine the .tax files into the same return but you can import or transfer them to separate returns.
Do you want to transfer from 2017 into 2018? Or open .tax2018 files? If you used the Online version last year I can tell you 2 ways to transfer them.
And there are several other features and benefits to using the Desktop program. You can buy it here,
https://turbotax.intuit.com/personal-taxes/cd-download/
Where did you get the .tax2018 files? Did you download it from the Online version?
After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW).
See, <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1901476-how-do-i-switch-from-turbotax-online-to-the-turbotax-software">https://ttlc.intuit.com/questions/1901476-how-do-i-switch-from-turbotax-online-to-the-turbotax-software</a>