I offer this solution in the hope that the methodology may help others in a similar situation:
A possible solution to the John Laidlaw children problem.
Since the fathers have the same name, it would be possible to distinguish the two families by the mother’s birth name - which is unrecorded! But also by the requirement (1) of at least 9 months between births to the same mother, and by the implausibility (2) of a family having two children living at the same time with identical single forenames. And the two different marriage dates (3).
The Lyne & Megget parish registers provide the following facts:
A John Laidlaw married Margaret Bryden on 31 May 1670. Referred to here as JL-B.
A John Laidlaw married Christen Lyntoune on 30 Dec 1675. Referred to here as JL-L.
Children registered:
Marion (F) 06 Mar 1670. (3) JL-B. Probably the reason for the marriage.
Christian Laidlaw (F) 19 Mar 1671. (3) JL-B.
William (M) 09 Mar 1673. (3) JL-B.
Walter (M) 02 May 1675. 7 months before the JL-L marriage, therefore PROBABLY JL-B.
Twins: John (M) 24 Dec 1676. James (M) 24 Dec 1676.
Robert (M) 16 Dec 1677.
Note the following sequence:
James (M) 01. Jun 1679. Issoble (F) 15 Feb 1680. William (M) 25 Oct 1680.
Applying (2) - William must be JL-L, since there was already a William JL-B.
Applying (1) - Issoble could not be the sister of James or William. But there were only 2 JLs recorded, so James (M) 01. Jun 1679 and William (M) 25 Oct 1680 were brothers, and James (M) 01. Jun 1679 was thus JL-L. Issoble (F) 15 Feb 1680 was therefore JL-B.
Applying (2) - since James (M) 01. Jun 1679 was JL-L, the twins John (M) 24 Dec 1676. James (M) 24 Dec 1676 were JL-B.
And finally: David (M) 14 May 1682, who cannot be assigned by the above rules, but because of the advanced date is PROBABLY JL-L.
Marion (F) 06 Mar 1670. (3) JL-B.
Christian Laidlaw (F) 19 Mar 1671. (3) JL-B.
William (M) 09 Mar 1673. (3) JL-B.
Walter (M) 02 May 1675. PROBABLY JL-B.
John (M) 24 Dec 1676. James (M) 24 Dec 1676. JL-B
Robert (M) 16 Dec 1677. INDETERMINATE
James (M) 01. Jun 1679. JL-L.
Issoble (F) 15 Feb 1680. JL.B
William (M) 25 Oct 1680. JL-L.
David (M) 14 May 1682. PROBABLY JL-L.