Liz, came across a couple of births which may help.
Births for Elizabeth Tait:
born & baptised 24 April 1782
Kirkmichael, Ayr
father: Hugh Tait
21 June 1787
Relief Congregation, Irvine, Ayr
father: James Tait mother: Margaret Weir
22 Dec 1784
Symington, Ayr
father: Thomas Tait
born: 1 March 1779 bap: 3 March 1779
Galston, Ayr
fater: John Tait
Births for John Thom:
bap 7 Sept 1783
Coylton, Ayr
father: John Thom mother: Agnes Murdoch
bap 20 Nov 1771
Tarbolton, Ayr
father: John Thom
born 9 Dec 1777 bap 14 Dec 1777
Old Cumnock, Ayr
father: James Thom mother: Janet Mitchell
born 15 Aug 1770 bap 16 Aug 1770
Ayr, Ayr
father: James Thom mother: Jean Mcfadyian
born 23 Dec 1779 bap 25 Dec 1779
Ayr, Ayr
father: James Thom mother: Margaret McKergo
going by their marriage cert, I would say that for John Thom the 1st one is most likely his as it is the same place he is from on cert. As for Elizabeth, it would be a matter of finding more information. Hope this helps, Kim