How do you keep up with the stacks? It's hard enough keeping up with .net changes and that ecosystem is much smaller than java.

You don't need to know every single detail.

Usually we switch stacks when we switch customer, on average every six months or year.

Sure there is a kind of adaptation period, like the first week, then is back again to full speed.

I also use both on personal projects, so sometimes I use the stack that I currently don't on the project.

There are also mixed projects where the frontend is a native desktop application written in .NET, with the backend done in Java.

Basically, it is no different of mastering several human languages at writing, reading and speaking levels.