There is no good solution to this since all files use the same .j2l extension. If you want, you could set your .j2l to automatically open TSF JCS, but then when you double click 1.23 levels if you save them, they will be in 1.24 format, which is a bad thing.

But, if you want to make your .j2l load TSF JCS automatically follow these steps:
1. Open Windows Explorer
2. View -> Folder Options
3. File Types
4. Find, Jazz 2 Level
5. Click Edit
6. Under the actions window, click Edit
7. Change the path to the JCS you wish to have it open.