Is "ni l'un ni l'autre" singular or plural?

Mes deux frères sont bons en langues, mais ni l'un ni l'autre ne sont bon en maths. 
My two brothers are good at languages, but neither is good at maths

Should this be "ni l'un ni l'autre n'est bon"? According to a french speaking friend (and Gemini) l'un and l'autre refer to each brother individually, so it should be singular. 

Thanks