Making **certain** that two concurrent instances of of R/RStudio open on the same computer do not interact

If I have two different instances of RStudio open on the same computer but to different projects, and I want to make sure that they do not interact in any way, at least for the duration of the session, I know that they should not:
read from or write to any common folder;
update packages without some system of package management;
make changes in .Rprofile or in any of the environment variables.

Is there anything else I need to do to ensure this? I am pretty sure I have seen crashing one instance of R/RStudio crash the other, probably by using up all the memory on the system or by freezing R (e.g by trying to open the viewer on a nine million line file. Although it can do that. By tomorrow)