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.