You have to remember that the people who make games think about these kind of problems all the time
You need to damage the Smash Ball to open it, right? I imagine that the person who deals the final blow on the Smash Ball gets the Final Smash.
In the unlikely event that two characters BOTH land the final blow at EXACTLY the same time, the choice would most likely be solved by totalling which character had dealt the MOST damage on the ball so far.
If, in the VERY unlikely event that both of these parameters are equal, some other 3rd parameter would be taken into account, such as which character has the most damage, etc.
The chance that 3 parameters are all equal is EXTREMELY unlikely. You can only introduce parameters so far, however, and you have to make a decision as to whether it is worth your time setting more parameters for a 1/1000000 chance, or whether to just make the choice random.
I believe, also, that only one Smash Ball is available at any one time.
Hope this clears things up!