Regarding seasons with two champions in one team:
Not counting future or almost champions, because then it's not a fair comparison to next year.
1953 - Ascari/Farina at Ferrari - Totally dominant F2 car. (Last time Ferrari had two champions by the way)
1967/8 - Clark/Hill at Lotus - The 49 was the fastest car by a long way.
1974 - Fittipaldi/Hulme at McLaren - The M23 was good enough for Emerson to win the title.
1985 - Lauda/Prost at McLaren - MP4/2B was the class of the field most of the year.
1986 - Prost/Rosberg at McLaren - MP4/2C still competitive enough for Alain to win the title.
1989 - Senna/Prost at McLaren - The MP4/5 was easily the best car.
2010-2012 - Button/Hamilton at McLaren - Not the best cars in general, but competitive enough for 18 wins in those three years.
Says a lot about McLaren's hiring policies really.