You can change the sound. You need to put the new sound file in a specific location/name (cannot remember, but there's one "addon" that does just this replacement so you can use it to find where the file should be located)

How does this help?

If you run each window from a separate installation, you can record 5 sounds saying "one", "two", .. "five" and put the files in the respective installation's filetree. You should also turn on the option to have sound from the client even when it's not the "in-focus" window, i.e. when it's in the background. The effect would be that when the bobber fires up you will hear your recorded sound telling you which window to "fish" it up from.

It's can get a bit busy as the timers are random, and you will miss some casts this way. But with the right maximizer setup you should be able to whack-a-mole fish pretty decently. Also use something like fishing buddy to make "easy cast" with double-right-click and it should be manageable.

That being said, I personally find that this method takes the soul out of fishing. Fishing is meant to be something peaceful that you do while half-afk (e.g. speaking on the phone, reading a forum, etc) This method requires a lot of attention and you will grow tired of it pretty quick. I have tried it but in the end I found it easier to level each toon individually. Takes more time, but you can use actual dead time to do it, no need to focus on it 100%.