Paul's use of the presence-absence dichotomy clarified that absence does not materially affect the Work of God because the same Holy Spirit is always present.
How do we 'sharpen' another's countenance? Most importantly, the imagery implies proximity, closeness. Nothing can be sharpened unless there is contact.
Honing a blade re-forms or re-aligns the blade, while sharpening removes metal to expose or create a sharper edge. This happens within friendship, too.