If your using the turret behavior then you can add certain targets to each turret,
on start of layout - 'tower1' add turret target 'enemy1'
and you can set bullet speed, rate of fire, predictive aim etc,
it will be more precise but aznmonkeyboy answer is the easiest :)
The turret behaviour is good for a lot of the tower defense games you see that use a radial targetting system. Ubru asked if there was a way to make it akin to Plants vs Zombies.
The problem with the Turret behaviour here is that the turrets will start target enemies on other lanes.
Ubru
I actually forgot one more step Ubru. Apologies.
Like L3mon said, you can differentiate them using variables.
1. Give both your 'towers' and the 'lines' an integer variable respective to their lanes. ( say for example {lineNum} )
2. If an enemy is overlapping said 'line', you can then compare and pick the 'towers' who's {lineNum} matches up with the 'lines' {lineNum}
3. then FIRE AWAY.
I'll see if I can whip up an example for ya.