|Chíonn beirt rud nach bhfeiceann duine amháin.||Two people see what one person does not.|
That Chíonn is an alternate form of Feiceann, found widely in the North, but also used by some Munster speakers.
It's a bit odd to see nach bhfeiceann in the same utterance. In class, I observed that I haven't run into forms like Nach gcíonn at all, but Dineen's dictionary says that that can happen.
Nonetheless, other sources indicate that modern usage is to use feic- forms with particles. That is, Chíonn sé,
but Ní fheiceann sé.