THE DUAL TEMPERATUR SYSTEM IS A HOTWATER SYSTEM ,WITH TO HEAT PLATE EXCHANGERS DIVIDING THE CHILL WATER SYSTEM TO THE HOTWATER SYSTEM. WHAT IS THE PURPOSE OF THE PLATE EXCHANGERS.6 INCH CHILL WATER PIPING, 8 INCH HOT WATER MAIN.
The plate exchanger could be separating the two systems because of the source pressure (such as in a high-rise) or because the hot or chilled water has different solutions (glycol or no glycol, chemical treatment, etc.
It could also be that the PHE is used for free cooling water-side economizer using condenser water for example.
Without a diagram or more information it is just a guess.
Traditionally, 2-pipe systems have 3-way diverting valves for change-over. Again, a diagram may help to get you a better answer.
"If you do not know the answer, say, "I do not know the answer", and you will be right!"