It is not a good practice as any external linking can lead way to file corruption.
If I read your post correctly you are needing to use both the start and finish dates from one schedule in another.
This can be accomplished by going to the start and finish cells of the task you want to use the start finsh date, copy these cells and go to the task in the file and cells where you want to use these dates, click in the cell and right click and paste special, and select link. These dates should now change in the copied file when ever you change them in the file from whence they were copied.
