This is a working research note, since I do not have a definitive answer. -- Wesley Johnston (last updated 16 May 2012)
The parish registers alone are not sufficient to resolve the question of which John Harper married which wife in the late 1700's -- or whether it was the same John Harper in both marriages.
Marriages
There were two marriages in Luxulyan, and both families wound up in Ontario:
1792-Jan-2 - John Harper and Mary Vicary, witnessed by John Roach and William Nanjulian
1796-Apr-28 - John Harper and Ann Cock, witnessed by John Nanjulian and William ?Beswetherrick?
Just to be sure, here are all of the other Luxulyan marriages of a John Harper found in the Cornwall Family History Society Research Database:
4 Feb 1662/3 - John Harper and Anne
5 Jan 1762 - John Harper and Ann Hawken
18 Jun 1822 - John Harper and Elizabeth Dally
18 Jun 1827 - John Harper and Grace Roach
So there was only one John-Mary couple, and there was only one John-Ann couple who would have been having children in the 1790-1810 period, unless another John Harper couple had moved to Luxulyan. Thus, we can take the list of baptisms and assume that they match the two couples in question.
Children
The Cornwall Family History Society Research Database lists the following children’s Luxulyan baptisms for these couples:
1792-Jan-2 - John Harper and Mary Vicary (6 children)
26 Dec 1792 - John
23 Jun 1793 - Richard
10 May 1795 - William
29 Jan 1797 - Mary
20 Nov 1798 - Joseph
22 Mar 1801 - Ann
1796-Apr-28 - John Harper and Ann Cock (9 children)
5 (or 6?) Jan 1800 - Richard
4 Apr 1802 - William
12 Feb 1804 - Matthew
7 Apr 1805 - Joseph
10 Jan 1808 - Walter Hoskins
18 Mar 1810 - Francis
4 Jul 1813 - Ann
11 May 1815 - Mary Cock
30 Nov 1820 - James
It does seem likely that John and Ann had a child before Richard who died as an infant or possibly was baptized in another parish. (Ann Cock was born in Luxulyan, so that another parish seems unlikely.)
The given names of the children do not seem to shed any light on who the parents of both John Harpers were. But the fact that the births overlap after 1796 rules out the possibility that these were two marriages of the same John Harper.
The middle name Hoskins of the 1808-baptized son of John & Ann is intriguing. It is not his mother's maiden name. But sometimes children received the maiden name of a grandmother as their middle name. Ann Cock's mother's maiden name was Davy. So if there was a Hoskins grandmother then it would have been the mother of John Harper
Candidate John Harper Luxulyan Baptisms
Assuming that the 1792-married John was no older than 50 and the 1796-married John was no younger than 17, then the two John Harpers would have been born sometime between 1742 and 1779. Of course, the John in one of the marriages could have been baptized in another parish. But what John Harper baptisms were there in Luxulyan in that period? There were only two.
20 Jun 1757 - age 35 in 1792 and 39 in 1796 and 63 in 1820 (birth of James) - son of Richard and Mary
25 Jul 1768 - age 24 in 1792 and 28 in 1796 and 52 in 1820 - son of John and Ann (must have been the 1762-married John Harper and Ann Hawken noted above)
Conclusions
Theoretically, either John Harper was of age to be the husband in either marriage. However, it is more likely that the 1757 John was the husband in the 1792 marriage (to Mary Vicary) and the 1768 John was the husband in the 1796 marriage (to Ann Cock). This alignment also puts the Hawken/Hawkins grandmother with Walter Hoskins Harper, giving a tiny bit of weight, since the names are not the same.
Having to represent this in one way or another, I am going to use this alignment in my tree until I find evidence that refutes it. This alignment will agree with some other trees on Ancestry and the Internet and disagree with others. But I have not seen any solid evidence or consideration of the question in any of those trees. So I am going to use the more probable alignment, recognizing that there is still a third possibility of a non-Luxulyan John Harper being one of the husbands.
Auxiliary Item: 1813 Luxulyan Death of 1768 John
I have seen a number of trees that show a 1 Oct 1813 Luxulyan death for John Harper. There is no such death in the Luxulyan records, neither in the Cornwall Family History Society Research Database nor in FamilySearch. Clearly such a precise date and place leads me to suspect that someone has seen such a record. But no one has presented it. I carry it on my tree, pending further research.
Were the two John Harpers related?
The 1757 John was the first cousin of the 1768 John. 1757 John was the son of Richard Harper (bap 23 Oct 1726 at Luxulyan) who was the brother of the John Harper (bap 23 Jun 1735 Luxulyan) who was the father of the 1768 John. The parents of the brothers (and other siblings) were Richard Harper (bap 14 Mar 1703 St. Juliot) and Elizabeth Rosevear.
There was also a link between them in the 1841 Whitby, Ontario marriage of Susan Harper, daughter of John Harper and Elizabeth Dally, and thus granddaughter of John Harper and Mary Vicary, if my alignment is correct. The witnesses at this marriage were Luther and Ann (Harper) Harnden. Ann Harnden was the daughter of John Harper and Ann Cock.