Formula to Add Two Cells Together with Seperate Formulas

Hello,
I need help with creating a formula. The result needs to be adding together two cells which have a multiplication for each. I want the results cell to have a blank result if it is zero. Here is what I have, but cell 890 and 891 are not adding their results together:
(note: cell 890 will be blank until I enter a value, and cell 891 will always have a fixed value).

Cell 890 and 891 will always be multiplied by the same value (9%), so maybe they should be added together first, then multiplied by 9%?

As this string now stands, the result cell is only showing the multiplication of cell 890

=IF(F890=0,"",(F890*0.09)+(F891*0.09))

Thank you