Second Tier
Pope Nicholas I is thought to have been the first to unite the princely crown with the white headcovering. However the common belief that Pope Boniface VIII (r. 1294-1303) around 1300 added the second crown, is disputed. While an inventory of the papal treasures in 1295 suggests that the tiara had at that stage only one tier, Pope Innocent III (r. 1198-1216) is represented wearing a two tiered crown in a painting that predates Boniface.
What is certain, from statues of Boniface made during his lifetime and which he saw (and so did not dispute the accuracy of), is that he wore a two-tiered tiara, so the two-tiered tiara originated no later than his reign. Two of these statues of Pope Boniface are in the crypt of St. Peter's Basilica. Why the second tiara was added is not formally stated in any documents, but historians such as James-Charles Noonan suggest that it may have been symbolic of growing papal claims to both temporal and spiritual power, the two tiers in the papal crown contrasting with the single tier of standard monarchical crowns.
Read more about this topic: Origins Of The Papal Tiara