What is the meaning of "two new and two old" in Feng Shui?

It is unlucky to clip the old with the new and auspicious with the new.

This is like, the houses on both sides are new, the house in the middle is old, and the house in the middle is high, so the transition from both sides to the middle and the transition from the new to the old are all bad transitions.

On the contrary, two old sandwiches are a good transition.