Adjust numbers (iWork app) formula to add if 2 cells are 0, leave blank
If I understand what you are trying to accomplish, the formula can be simplified to this:

IFERROR(IF(OR(A2,B2),A2+B2,""),"")

Since 0 resolves to false and OR takes Boolean values, just check for 0 in both cells and return the empty string.  (If both are false then the true event is not evaluated.)  Otherwise, if both are empty, the IFERROR will return the empty string as, for some reason, an empty string in Numbers is not treated as a Boolean false like in programming languages such as JavaScript.  

See the example below.  This will work in Numbers on macOS or iOS.