That the two are essentially one and the same, with respect to the existence of free will at least, is obvious, and testified to even by Calvinists. (Not with respect to saying you have to believe in Jesus and be baptized to be saved, which Pelagius most certainly does say in his commentary on Romans).
From an article, Who’s afraid of Pelagius? (in the comments):
(Pelagius famously claimed that Adam would still have died had he never tasted the apple—common sense sort of guy, that Pelagius—and denied original sin as well).
From another article, Pelagianism, Hyper-Evangelism, and Hyper-Calvinism 10 (the very first sentence):
The basic points of Pelagianism may seem like common sense to many people, but that simply points to the need for humanity to recognize its fallen condition. What seems like common sense is simply natural sense or the sense that comes from sinful human nature.
So there you have it from the Calvinists’ mouth: Pelagianism is common sense, but common sense is a sin.
And here’s a really good one, Infant Baptism, Pelagianism, Good and Evil:
“Cranmer required that baptism be administered freely, or to use a weasel word popularised in the 1960’s, ‘indiscriminately,’ to babies…
…Cranmer’s baptismal liturgy, above all, is a trenchant expression of western Augustinianism. It rejects utterly the Pelagianism that has fascinated the English since the fifth century. Pelagianism indicates how God ought to work if he had any common sense.“
I think I could amuse myself for days just collecting all the occurrences of Pelagianism and common sense together on the Internet, and I may very well do that.