How should MAXDOP be configured on stacked instances?

There is good general advice for how to configure Maximum Degree of Parallelism (MAXDOP). However, I am in a special circumstance. There are two instances on my box. If I have 16 cores, how should I divide MAXDOP between them? What metrics would reveal that I have made a bad choice? I am on SQL Server 2019 and [the standard advice for MAXDOP changed in 2016](https://learn.microsoft.com/en-us/sql/database-engine/configure-windows/configure-the-max-degree-of-parallelism-server-configuration-option?view=sql-server-ver16#recommendations) so info from before June 2016 (the release date of SQL Server 2016) may not be applicable.