Joyce has an interesting take on the question of time and times:
Television kills telephony in brothers’ broil. Our eyes demand their turn. Let them be seen! (FW 52)
Exactly contrary to McLuhan’s usual practice (although not to his intent), vision is here taken in synchronic fashion (since vision can see difference “all at once”) and hearing is taken diachronically (since hearing can hear difference only over time, first one sound and then another, “one at a time”).
The doubling (Dublin) of vision here — Let [our eyes] be seen! — implicates simultaneity (aka “double perspective“) as method: a method to be used in understanding vision as much as the relation of vision to hearing as much as the relation between the brothers in their broil. “Let them be seen!”
It would seem/seen that synchrony and diachrony, vision and hearing, are each both.