working on same return on 2 different computers using same downloaded program. How to do this?

That will turn into a mess.    When you use the download software the tax file is only stored on your own hard drive.    If you want to go back and forth between two computers  to work on the same tax return, you will have to keep saving the file to a back up device like a flash drive and keep putting the changes into the return each time you switch back and forth.

presumably doable tho if you have a cloud drive accessible to both computers and set up the .tax file there