I recently gave a demo of my FM software to a future user and ran into problems I never did before, problems that wouldn't have come up under normal circumstances: error messages from FM or a window that should have remained hidden replacing the window that should have remained on display. (I wasn't able to take screenshots or record details at the time.) I tried to reproduce the problem later but was not able to. It did strike me that the one thing unusual about the situation was that I was also demonstrating another completely separate database, written by somebody else, that the user would also be using in the future. My question is whether having two databases open at the same time can cause unexpected things to happen in one of them? Is it therefore consistent with best practices to operate only one FM database at the time? Or are there ways to write scripts, in particular to manage window, that can keep the database insulated?

If two databases cannot interfere with each other, then I will have to assume that there is some hard-to-reproduce bug in my code.

Were they using their own copy of FileMaker? or your copy?

I'm not sure I understand the question completely, but here's more about the situation: The user ran his own copy of FM (13.0v9), which I recently helped him install, on his Mac, while I screen-shared from my Mac. The two databases were running remotely on separate servers at different locations on my university's network. The databases on the two servers were up to date and stable. 

There is nothing inherently bad or wrong about having multiple FM files / different solutions open at the same time.  People do it all the time.

There is however the possibility of accidentally doing something that causes unwanted interference between the two.  Say that solution A has a hidden window named "temp" and a script in solution B has a script step to select a window named "temp" with the option to only select windows from the current file turned off: it could select that window from solution A.

It sounds, then, as though the problems were probably due to a bug, but with a very small possibility that the two databases were interfering with each other. Thanks for the responses.

I have two versions of FMP open at the same time to test doing updates, and make instructions on how to do the updates at the same time.  On a MAC you simply open up the terminal and type this,     open -n "/applications/filemaker pro 16/filemaker pro.app"

That will open up another version of FMP.  Enjoy!