Why should different version of Sybase on a single Windows server be an issue? If you install multiple versions of Sybase ASE, the paths will become filled with contradictory references to both
Sybase install locations. The Windows servers may or may not start but even if they do, you cannot be 100% sure which library paths you are using for the Sybase ASE server, so you could encounter support issues later down the line.
You can work around this by starting Sybase from a command window (MS-DOS), run the SYBASE.bat, followed by the RUN_SERVER.bat. However, the ASE server will need this window to remain open for it to keep running and any accidental closure will cause the ASE server to shutdown.
The solution attached shows how to run the ASE servers in the background, as a service.