This is what I have
1841 Census - Helebaigh, Coylton
David McFADZEAN aged 60 (Ag.lab)
Margret wife 55
Margret daughter 15 David son 5
(This seems a bit strange, but perhaps Margret was not as old as she thought she was).
There is a birth registered to David McFADZEN and Margaret Dickie (Margaret) at Coylton, 26 August 1822 which is probably this couple.
1841 - Drumdow Colliery, Stair
David McFADZEN aged 32
Agnes 40
Mary 12
Hugh 10
David 8
Ann 1
(This is from FREEcen and no relationships are given. However, I have seen the original of the first family on the census in Ayr Library)
The second family could be David McFadzen and Agnes Baird who married in Coylton 1 June 1827.
IGI Batch numbers C115832, C115834 and C115831 (various spellings of McFadzean) give quite a few details of McFadzen/McFadzean births in Coylton. Marriages are the same numbers but with an M in front.
My ggxgrandmother was Margaret McFadzen, (abt. 1797) sometimes McFadzean, who married Peter McCormick in Coylton in 1822. Her parents (as per death cert.) are James McFadzen and Janet Paton. She had a sister Janet born 18 May 1781, Coylton. The first David on this list may have been their brother, or possibly an Uncle. Have not investigated much more on this family.
Hope it gives you some sort of lead.