Instantiate between 2 game objects.

The simple version of Instantiate() takes only one argument: the object to copy.

There's a three-argument form that takes the object to copy, then the target position and rotation for that object.

Something like this:

 var fooClone = Instantiate(foo, Vector3.zero, Quaternion.identity);

Now, let's suppose you have two points, v1 and v2. You'd like to spawn an object between them.

 //random point between v1 and v2
 //you already have this, but someone else might read this thread...
 Vector3 pos = Vector3.Lerp(v1, v2, Random.value);

 //vector pointing from v1 to v2
 Vector3 separation = v2 - v1;

 //rotation facing from v1 to v2
 Quaternion rot = Quaterion.LookRotation(separation);

 //spawn
 GameObject copy = (GameObject)Instantiate(someOtherGameObject, pos, rot);

Handy reading:

• http://docs.unity3d.com/Documentation/ScriptReference/Quaternion.LookRotation.html

• http://docs.unity3d.com/Documentation/ScriptReference/Vector3.Lerp.html

• http://docs.unity3d.com/Documentation/ScriptReference/Random.html

• Google "vector subtraction" if the separation formula seems unusual

For the object you are putting in the middle, you have to pick a side you are going to have face along the line. So given you are saying the rotation along the 'z', I'm going to assume you want the right side of the object to face the second square. Given the Vector3 positions of the two squares as 'pos1' and 'pos2', you can align the right side as:

 var v3 = pos2 - pos1;
 transform.rotation = Quaternion.FromToRotation(transform.right, v3) * transform.rotation;

Another solution is to fist construct your object you are placing so that the side facing positive 'z' is the one you want to point at block 2 (pos2). Then you can do this:

 Transform.LookAt(pos2, Vector3.forward);