Most likely they were hundreds of miles apart from one or the other and picked up a contact on sonar, classified it as a submarine and went to investigate
Seems most
unlikely to me since, certainly in the normal course, the primary aim of submarines on deterrent patrol is surely to remain undetected, and not to go looking for trouble. VANGUARD was on such a patrol, whilst LE TRIOMPHANT was on passage.
Maybe if they put windows in the front of them then the sub drivers could see where they are going
Curiously enough, a well known submarine cartoon shows such a window, albeit in the side of submarine, marked "In case of fire break glass"!
Jack