Hmmm.... in the init.sqs I think you can specify which radio commands will be visible to different players.
?!(player == playerName): 0 setRadioMsg "NULL"
?!(player == playerName): 1 setRadioMsg "NULL"
?!(player == playerName): 2 setRadioMsg "NULL"
?!(player == playerName): 3 setRadioMsg "NULL"
?!(player == playerName): 4 setRadioMsg "NULL"
?!(player == playerName): 5 setRadioMsg "NULL"
That means that if you are not player playerName you won't be able to see those commands. However... I don't think you can have two or more players share the same command number in the list. But of that I'm not certain.
I would give the first, say three numbers, to the first player and the three next to the second player.
Regarding what you asked about a specific player activation a script by radio command, you could make the script run different sections by checking who has activated it:
?(player == playerName1): goto "playerName1stuff"
?(player == playerName2): goto "playerName2stuff"
I might be wrong about all this though.
This is just stupid suggestions from a stupid n00b but I hope some of it helps ;D