You will want to specify the object by either using a tag comparison or name comparison. Below is an example using tag(since if you clone the cube its name will go from "Cube" to "Cube(Clone)" thus preventing the code from continuing..

function OnTriggerEnter(other:Collider)
{
    if(other.tag=="BridgeCube")
    {
        myBridge.collider.enabled=true;//turn on collider
        myBridge.renderer.enabled=true;//turn on renderer
    }
}
function OnTriggerExit(other:Collider)
{
    if(other.tag=="BridgeCube")
    {
        myBridge.collider.enabled=false;//turn off collider
        myBridge.renderer.enabled=false;//turn off renderer
    }
}

Not 100% sure if it works... but I'm thinking it will. Adjust the timer to 0.1 if you want instant release, if not.... Poof. You've got a timer.

var Bridge : GameObject
var Timer : float = 1;

function Update(){

var TimerUser
//Timer Counts Down
TimerUser -= Time.Deltatime;


//Stops from going below 0
if(TimerUser < 0 )
TimerUser = 0;


//If there is time left, Bridge is active, else it is not active
if(TimerUser > 0)
Bridge.active = true;

else
Bridge.active = false;


}


function OnTriggerStay( Col: collider) {


if Col.GetTag("Box")
TimerUser = Timer;

}