hmmm... i think it is simply a matter of manually firing at one ship, click on "take secondary target" then choose second target and start firing on that one (repeat steps for 3rd target, 4th, etc)... you should be then firing at your new target and your previous target(s)...
if thats not working, please feel free to make a new thread about it and perhaps whatever is going wrong can be fixed...

or, you can use FTech; USS Frontier made a script that will allow a ship to fire on 2, 3, 4, etc etc ships at once at all times via the plugin...
(and i think the tech can be turned on and off ingame)