Duncan twins married Cecil Turk and Granville Jerrard

[Stuart P]

Agnes Morrison Duncan and Isabella Morrison Duncan were born 21 Mar 1912 at Ross Cottage, Lumsden, Auchindoir, Aberdeenshire to Isabella Dawson Duncan [b.11 Sep 1886 Cabrach, Aberdeenshire]. The mother's Scottish descendants are vaguely aware of twins who were born and were supposedly adopted in England. The 2 girls both married at Battle reg. dist. in the 1930s under their original names - Agnes M. Duncan to Cecil Macdonald Turk in Q4 1935; Isabella M. Duncan to Granville Owen Henry Jerrard in Q4 1937.
Agnes is in the 1939 register with her husband at Pembury Grove, Bexhill-on-Sea and has given her correct birth date [this is how I found the twins in England on Findmypast].
The query is: Where were these girls as children? There were no statutory adoption laws in 1912, and they appear to have kept their names.

[CaroleW]

Do either of their respective marriage certs show their "adoptive" fathers name? 

If not - the only hope would be the address they give on their marriage certs - which may not necessarily be the same for both of them.  You could check the electoral registers immediately prior to the marriages to see who was living at that address.  The twins will only appear from when they were 21

[Stuart P]

You could check the electoral registers immediately prior to the marriages to see who was living at that address.  The twins will only appear from when they were 21

I haven't seen any online source for 1930s East Sussex electoral registers. I suppose they would be held in local libraries, but not indexed by surname?

Checking the Bexhill Observer, I see there is a J.B. Duncan Esq. at 16 Knole Rd, Bexhill in 1935; a Miss Duncan as a girl guide leader in Bexhill in 1930.

[CaroleW]

The address on the marriage cert is what I was suggesting you check on the electoral register.  You would have to visit the records office or library in that area or email them to see if they would do a lookup for you