To complete and file a 2017 and 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the desktop editions for each tax year from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2017 and 2018 tax return can only be printed and mailed, they cannot be e-filed. Use a separate envelope for each tax year when mailing the tax returns.