Can I do my taxes of 2018 and 2019 together or is it okay for me to do them separate?

[macuser_22]

You MUST do them separate.  

TurboTax online and e-file for 2018 ended on Oct 15, 2019. 2018 (and earlier tax years) can now only be completed using the CD/download software installed on a Mac or Windows computer. The return must be printed, signed and mailed.

2016-2018 software can be purchased here:
https://turbotax.intuit.com/personal-taxes/past-years-products/

[DoninGA]

Each tax year must be filed completed and filed separately.  The TurboTax online editions are Only for tax year 2019.

To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/

A 2018 tax return can only be printed and mailed, it cannot be e-filed.