Is it possible to use a single Time Machine backup so that two Macs are always in sync? I have a Time Machine and two Macs.  I want both computers to be the same, not back them up separately. 

Is it possible to use one as a primary (always backing up from one) and then "downloading" the latest changes onto another?
Time Machine is not designed to do this. It will not make bootable backups, so you can't use a Mac as a destination for backup (and have this backup be "functional").

There are other solutions for synchronizing multiple machines. If iCloud/Dropbox/Google Drive type solutions are not sufficient for your needs, there are other tools that can sync - Chronosync is one solution.