Here you have two schools really. One says that you should keep your privates private, meaning you should have different accounts for different applications. SharePoint 2016 is a different application than SharePoint 2013. It makes things easier if you for some reason need to add SPN to the SP_ProfileSync or SP_AppPool accounts in one environment, but not in the other. So when we come to production and acceptance test environments, its recommended that you use different accounts.
But in my dev environment which consists of three machines; (1) Domain Controller, (2) SQL Server and three different SharePoint Servers (2010, 2013, 2016) I use the same account for simplicity. I even use the same database server (and instance) for all three environments separated by a db-prefix SP2010, SP2013 and SP2016. It works very well, and it’s fast to configure a new farm if I need to, since all setting for the SP_ProfileSync account is already there.
But this doesn’t come from any official documentation, just from my own experience.