I want to create an enemy that spawn when the level starts and then increase over time. However, with the code I have right now the objects will never actually appear in my game. And if I move the Instantiate to if(spawnTime == 0) then two of each object will spawn (only 1 of each should).

var spawnTime : float = 0;
var timeIncrease : int = 0;
var Enemy: Transform;
var Enemy2:Transform;

function Update(){
    if(spawnTime == 0){
        timeIncrease = Time.time;
        spawnTime = Time.time;
    }

    if(spawnTime == 10){
        Instantiate(Enemy,transform.position,transform.rotation);
        Instantiate(Enemy2,transform.position,transform.rotation);
        spawnTime = 0;
    }

    if(timeIncrease == 30){
        timeIncrease = 0;
        spawnTime = -0.5;
    }

}

SpawnTime is a float and you are comparing it to an int which isn't a good practice. Because spawnTime is a float and is incrementing based on the built in time class, its never going to be exactly equal to 10, therefore, it will never Instantiate your new objects. You could use a typecast to fix this, but its probably best to rewrite your code so you can keep the precision required with time.