The proof that there were two Alexander Yorkston(e) of similar age is in the 1841 census
1841 Census: Greenside Row, Edinburgh, Midlothian 685/4/2
Alexander Yorkston 27 Smith Journeyman Scotland not Midlothian
Catherine Yorkston 25 Midlothian
John Yorkston 4 Midlothian
James Yorkston 18 months Midlothian
1841 Census: Larbert Village, Larbert, Stirlingshire 485/1/2
Margt Nicol 34 Stirlingshire
Alx Yorkston 25 Mason Stirlingshire
Ann Yorkston 20 Stirlingshire
Janet Yorkston 1 Stirlingshire
We have one living in Edinburgh and giving his age as 27, which, if accurate, means that he was born in 1813 or 1814. The one in Larbert gives his age as 25, but since ages in 1841 were supposed to be rounded down to the nearest 5 years, he would have been born between 1811 and 1816.
Ten years later
1851 Census: Allison Place, Edinburgh St Cuthbert's, Midlothian 385/1/36
Catherine Yorkston Head Wid 42 Pauper Midlothian, Edinburgh
Alexr Yorkston Son 16 Engineer apprentice Midlothian, Edinburgh
John Yorkston Son 14 Message boy Midlothian, St Cuthbert's
James Yorkston Son 12 Scholar Midlothian, Edinburgh
Cathrine Yorkston Daughter 8 Scholar Midlothian, Edinburgh
Archibald Yorkston Son 6 Scholar Midlothian, Edinburgh
May Ann Yorkston Daughter 4 Midlothian, Edinburgh
William Yorkston Son 2 Midlothian, St Cuthbert's
This ties in with the 1850 death in Edinburgh.
1851 Census: South Broomage Hall, Larbert, Stirlingshire 485/8/9
Alexander Yorkston Head Mar 36 Farmer employing 3 labourers Stirlingshire, Larbert
Ann Yorkston Wife Mar 32 Stirlingshire, Larbert
Janet Yorkston Daughter 11 Scholar Stirlingshire, Larbert
Alexander Yorkston Son 9 Scholar Stirlingshire, Larbert
John Yorkston Son 6 Scholar Stirlingshire, Larbert
Peter Yorkston Son 4 Scholar Stirlingshire, Larbert
Ann Yorkston Daughter 2 Stirlingshire, Larbert
3 servants
This shows that the husband of Ann Kincaid was born in Larbert in 1814 or 1815.
So there were definitely two Alexander Yorkston(e)s born about 1814, one married to Ann Donaldson and one married to Catherine Phair/Fair.
The husband of Catherine Phair in Edinburgh is variously described as engineer, mechanical engineer and practical engineer.
The husband of Ann Donaldson is variously described as a mason, bricklayer and also a farmer.
The children of Catherine Phair/Fair were Alexander, John, James, Catherine, Archibald, Mary Ann, William and James, Catherine's parents were John and Mary. If they had followed the naming tradition their first daughter would have been named Mary, so no inferences can be drawn from her children's names.
The children of Ann Donaldson were Janet, Alexander, John, Peter and Ann. Ann Donaldson's parents were John Donaldson and Janet Malcolm. She named her first daughter Janet and her second son John, which is in accordance with the naming tradition. The names of her first son and second daughter are consistent with her husband's parents being Alexander and Ann.
So there definitely were two Alexander Yorkston(e)s of the same age
One born in Larbert about 1814, married in Larbert in 1839 and with children's names that would fit with his parents being Alexander Yorkston and Ann Kincaid.
The other born about 1814 somewhere in Scotland, married to Catherine Phair/Fair in Edinburgh in 1833, with children's names that cannot be matched to the naming tradition.
So it is still not possible to say with certainty which Alexander Yorkston was which, unless someone else has a piece of evidence to prove the identity of one of them one way or the other.
I don't really care very much what the correct answer is, but I do very much care about it actually being right!