Let's check the point we agree (please tell me if it's not the case):
- We can not play altar 2 only in response of the trigger "unit 2 is going to be destroyed"
- We can not play altar 2 only in response of the previous altar (because activation of an altar is not a trigger for activating an altar)
Do we agree that we automatically play the second altar in response of the first one?
The point we disagree :
- The rules allow you to play an action B in response to a trigger C and to an action A (both coming from different card and effect), if the trigger C is a correct trigger for Action B.