« Reply #1 on: Thursday 04 September 08 17:29 BST (UK) »
It just means that only one was born within wedlock. The other would not, at that time, have been recognised as a child of the marriage.
So far as English law is concerned, not until the Legitimacy Act of 1926 did the subsequent marriage of parents legitimate children born to them before the marriage.
Anna

Ayr: Barnes, Wylie
Caithness: MacGregor
Essex: Eldred (Pebmarsh)
Gloucs: Timbrell (Winchcomb)
Hants: Stares (Wickham)
Lincs: Maw, Jackson (Epworth, Belton)
London: Pierce
Suffolk: Markham (Framlingham)
Surrey: Gosling (Richmond)
Wilts: Matthews, Tarrant (Calne, Preshute)
Worcs: Milward (Redditch)
Yorks: Beaumont, Crook, Moore, Styring (Huddersfield); Middleton (Church Fenton); Exley, Gelder (High Hoyland); Barnes, Birchinall (Sheffield); Kenyon, Wood (Cumberworth/Denby Dale)