There's probably a registry setting that associates a folder with an .exe name, so rename each .exe. My Thief2 folder has Dromed.exe, and my ThiefG folder has DromedG.exe.
I use the startup.cmd file to run the load_mission command without any file. This brings up the Open window so I can be sure I won't accidentally save.
Checking new objects
If I want to have a look at a custom object without mixing its files with existing ones, I create this folder structure:
Thief2\something\obj\ (+txt or txt16 for textures).
In Darkinst.cfg I add the following to the end of the resname_base line:
+.\something
I then extract the object to Thief2\something\obj. I can look at it in Dromed while keeping its files isolated. Some objects come with lots of textures, or their names are obscure, so this makes it easy for me to keep my main obj folder clean. If I like the object, I can transfer it, and if not I can delete all of its files.
Posting on this forum
We all know TTLG sometimes has issues, and I tend to notice these while I'm waiting for a reply to be posted. Before submitting a post, I copy it to the clipboard in case the forum dies. Saves typing it out again.