Our Rabbis taught: If a man said to his wife in the presence of two witnesses, 'Here is your Get, on condition that you look after my father for two years', and he subsequently said to her in the presence of two witnesses, 'Here is your Get on condition that you give me two hundred zuz', the second statement does not nullify the first, and she has the option of either waiting on the father or giving the husband the two hundred zuz.
If, however, he said to her in the presence of two witnesses, 'Here is your Get on condition that you give me two hundred zuz', and he subsequently said to her in the presence of two witnesses, 'Here is your Get on condition that you give me three hundred zuz', the second statement nullifies the first.
Nor can one of the first two witnesses and one of the second combine to form a pair [testifying that there is some sort of condition].
The Gemara goes on to ask:
To which ruling [does this last statement belong]? It cannot be the second one, because [the first condition there] is nullified? Rather it is the first one. But in this case it is self-evident? — You might think that all [the witnesses who can help] to establish that there was a condition can be joined together. We are therefore told [that this is not so].
Rashi explains the first issue saying,
אילימא אסיפא - פשיטא דאין אחד מן הראשונים כלום שהרי אפילו באו שניהם אינן כלום דהא בטיל ליה ההוא תנאה.
It's obvious that one of the first set of witnesses can't count for anything, since even if they were both here their witness wouldn't be worth anything being that the first condition is already nullified.
And now, for the question
Rashi says there that even if both of the first witnesses were here it would be meaningless. But, if we only have the first two witnesses, informing us of the original condition, how would we know of the second condition which overwrote the first?
And if we know through the second pair then we already know about the condition and there is no further purpose for the whole conversation.