Excessive Vibration at Exhaust Fan

My query as below:

Fan Installation & Control

1. Two centrifugal exhaust fans have same fan static pressure (6"WG) & capacity (20,000 cfm) are placed in parallel.

2. Each exhaust fan has own individual outlet duct which is connected to individual chimney.

3. Each exhaust fan inlet shares same duct plenum which is further connected to main duct & branch ducts at production floor.

4. The duct plenum & main duct are sized for 20,000 cfm only.

5. Each exhaust fan motor speed is variable speed & controlled by Variable Frequency Drive (VFD).

6. Pressure transmitter installed at main duct is used to change the fan motor speed when there is a change of exhaust demand at branch ducts.

Scenario 1

1. Exhaust demand is less than fan capacity (15% of fan capacity).

2. When one exhaust fan is running, acceptable vibration at exhaust fan, chimney & duct.

Scenario 2

1. Exhaust demand is less than fan capacity (15% of fan capacity).
2. When two exhaust fans are running, one of the fan brake horse power is half of the another one.

3. Acceptable vibration at exhaust fan, chimney & duct.

Scenario 3

1. Exhaust demand is less than fan capacity (15% of fan capacity).

2. When two exhaust fans are running & have same brake horse power.

3. Excessive vibration at exhaust fan, chimney & duct.

My question is could someone advise me what has happened to the system ?