It sounds like you are adding chemistry labs and their ventilation to a building that already contains biology labs and their ventilation.  Biology labs may not have been given quite as robust a ventilation system as chemistry labs would need. Adding on may require substantial upgrades to the exhaust equipment, reassessment of the system’s exhaust points and stack height, and will certainly require careful design by a ventilation engineer who has knowledge of and prior experience with laboratory ventilation.  Do not leave it to the architect, or to just any design engineer.

The only issue I can see with stacking both floors on the same system is the need to ensure against backflow where effluent air from one department might be transported into the other department..

Peter Zavon, CIH
Penfield, NY

PZAVON**At_Symbol_Here**Rochester.rr.com