When was the Catholic Church undermined by the election of 2 popes?

Answer 1

This was called the Western Schism or the Great Schism and was from 1378 to 1417, see the Catholic Encyclopedia article at the link below.

from A Catholic Dictionary, edited by Donald Attwater, Second edition, revised 1957

The Great Schism, otherwise know as the Schism of the West was not strictly a schism at all but a conflict between the two parties within the Church each claiming to support the true pope. Three months after the election of Urban VI, in 1378, the fifteen electing cardinals declared that they had appointed him only as a temporary vicar and that in any case the election was invalid as made under fear of violence from the Roman mob. Urban retorted by naming twenty-eight new cardinals, and the others at once proceeded to elect Cardinal Robert of Geneva as Pope Clement VII, who went to reside at Avignon. The quarrel was in its origin not a theological or religious one, but was caused by the ambition and jealousy of French influence, which was supported to some extent for political reasons by Spain, Naples, Provence, and Scotland; England, Germany, Scandinavia, Wales, Ireland, Portugal, Flanders and Hungary stood by what they believe to be the true pope at Rome. The Church was torn from top to bottom by the schism, both sides in good faith (it was impossible to know to whom allegiance was due), which lasted with its two lines of popes (and at one time three) till the election of Martin V in 1417. It is now regarded as practically certain that the Urbanist popes were the true ones and their names are included in semi-official lists; moreover, the ordinal numbers of the Clementine claimants (who, however, are not called anti-popes,) were adopted by subsequent popes of the same name.