I have the best solution out of all of these:

1. Open Windows Explorer
2. Go Tools -> Folder Options
3. Go File Types
4. Find "Jazz2 Level" in the list and select it
5. Click Edit
6. Click New
7. In name, type "Open in TSF JCS" (without the quotes ")
8. In action type 'c:\games\jazz2TSF\jcs.exe "%1"' (without the single quotes ' but including the double quotes ")
9. Click OK
10. Click Set Default on whichever one you use the most
11. Click OK and OK again.

Now, whenever you want to open something in TSF JCS, right click on the file and choose "Open in TSF JCS". To open in 1.23 JCS, just double click it. Or if you set the TSF JCS to the default, then just do the opposite (you may want to have renamed "Open" to "Open in 1.23 JCS")

You can also add more actions, like "Play in 1.23" and "Play in TSF". Just follow the same process as above, having the path names 'C:\Games\jazz2\jazz2.exe "%1"' and 'C:\Games\jazz2TSF\jazz2.exe "%1"' respectively (without the single quotes ' but with the double quotes ")