Is it a good practice for teaching assistants to sit in the lectures?
I have seen TAs do this, but generally don't ask my own TAs to do it. As you say, it's a (very) time-consuming way to solve the coordination problem. I personally prefer to just sit with the TAs for a few minutes (&lt; 30 minutes) every so often to coordinate, which in my experience has been completely adequate. Of course this means that I have to invest a little of my own time, but I find it completely appropriate to invest a few minutes of my own time every so often to save my TAs quite a lot of time - but of course your milage may vary with other professors. I certainly wouldn't find it strange if a TA sat in the lecture, I just don't find it necessary.

However, I do strongly suggest that the TAs take some time to go over the teaching material, so that they get a sense for what I am covering in the lecture, and how far I have gotten by the time their tutorial or supervision session rolls around.

Finally, we have also used a joint Slack channel where TAs can ask questions and expect a fairly immediate response. This has been useful for me to communicate if I, for instance, needed to skip a section due to time reasons, or if I felt through in-class exercises (or blank stares) that a certain topic needs deeper exploration in the supervision sessions.