Post updated with info about challenge layers.
For reference, here are the affix combo chances for a typical green MI dropped from a Boss (or BossChest, in SR) on Ultimate difficulty:
No challenge layer
magic/magic 10.29%
magic/rare 44.49%
rare/magic 44.49%
rare/rare 0.74%
Easy dangerous domain
magic/magic 9.25%
magic/rare 45.00%
rare/magic 45.00%
rare/rare 0.75%
Hard dangerous domain
magic/magic 7.54%
magic/rare 45.85%
rare/magic 45.85%
rare/rare 0.76%
Roguelike dungeon
magic/magic 2.48%
magic/rare 48.28%
rare/magic 48.28%
rare/rare 0.97%
Boss chest in SR
magic/magic 2.48%
magic/rare 48.35%
rare/magic 48.35%
rare/rare 0.81%