That's a good question. The 2 cousins named Jane Scarff are less than 4 months apart.

For the Jane d/o John Scarff & Ann Kelly, I have her marrying John Radcliffe 21 Jan 1816 Andreas, with 10 children born up to 1838 all Andreas.

So why could they not be the other way around? There are no wills to help us.

One clue might be this 1841 census:
1841 census at Reaby [Rabey, nr Baljean] Lonan HO107/1464/2 36 pg 10 (464356)
Ann SCARFFE 70 Pauper Y [indexed SCAFFE]
Elizabeth SCARFFE 35 Y
Isabella SCARFFE 8 Y

I think the 70yo Ann was Ann Kelly, widow of John Scarff. There is a Lonan burial of a "Widow Scarff a pauper aged 80 years" for 11 Aug 1848.
The 8yo Isabella Scarffe is the illegitimate daughter of the 35yo Elizabeth, baptized 16 Sep 1832 in Andreas. Was Elizabeth staying with her sister Jane in Andreas in 1832?

Another clue might be one of the sons of John & Jane Radcliffe (nee Scarff), Enos b 1830 is in the 1861 census at "Cottage on Lemayne" Maughold right next door to James & Eleanor Kermeen. Eleanor was the youngest daughter of John Scarff & Ann Kelly, b 1816, md James Kermeen 7 Dec 1850 Braddan.

So, this places the Radcliffe Jane to be the daughter of John & Ann, so the other Jane belongs to Thomas & Eleanor. It's not perfect, but it's fairly strong I think.