I have to say I am as puzzled as you are by these apparent Twins,Martha and Margaret Taylor and the non appearance in the same Quarter and Year in the GRO Birth Index.Martha Taylor appears in the October Quarter of 1844,and there are two Margaret Taylors in the October Quarter of 1845.
Coming back to the `twins`
Dec 13 1843 Margaret and Martha were both baptised at St Giles Wrexham
daughters of Mary Taylor, no father named. They are in seperate entries .
In Isycoed Marriages 1863 Apr 11.
John Taylor 24 b Red Wither, father Edward Griffiths
Ann Mottersham 22 sp Isycoed. father Sam Mottisher (Mottershead ? )
Isycoed Baptisms
1863 June 7 Mary d/o John and Ann Taylor. Isycoed.
1865 Feb 25 Ermine ?? “
1867 May 19 Joseph "
This
John Taylor is the correct age for John with Elizabeth in 1851. As John was born before the `twins`, and Mary Taylor marries Edward Griffiths in 1852 at St Giles Wrexham, it looks like Edward Griffiths could be also the father of Margaret and Martha.

Before you all point out that the baptisms of the twins seem to be before the possible birth index entries,

I can`t find then in 1843 so will re-check again when I`m in the Museum..
