You can, but if you've already begun your return in the desktop version, it is not an easy process.
The following comment from TurboTax superuser "Steamtrain" suggests "You really need to work it out with the desktop software....it really should work for you better than Online. For Online you would need to start all over and upload your 2017 datafiles for each. (cannot upload the 2018 data file) AND...the Online software will require an entirely separate account for each.