Malbim: The form of a sword and knife is the sharp blade. When they want to sustain the form, they sharpen the iron on another iron and remove the rust. So is a man with his friend's face and form. Via engaging in Chachmah together, one makes his friend understand - 'I learned much from my Rebbeyim, and more from my colleagues' (Ta'anis 7a). They sharpen their faces, which are their form. Man's form is his intellect, Chachmah and powers of his Nefesh.

Rashi: Chachamim together sharpen each other in Halachah.