Range Vent Hood Ducting - Can two 5" ducts work in place of one 10" for a range vent hood?
Here are the numbers and you can see what your options are.

The numbers are as follows:

On the left: CFM (Cubic Feet per Minute, cfm)

On the right: Equivalent Diameter Round Duct Sizes

200 : 4.9

300 : 5.7

400 : 6.6

750 : 8.3

1000 : 9.1

1250 : 9.8

1500 : 10.7

1750 : 11.5