Y'okay,
Good and Bad news.
Bad...
I have no access to Dundee Records so can't help with the Kennedy marriage.

Good...
To make up for the above disappointment I used a few credits on Scotlandspeople to get your Twaddle/Young conundrum sorted. PM your email and I'll send you the image but here are the details...
When, Where, and How Married1866
On the fifteenth day of June at Calton Close Sanquhar After Banns according to the form of the forms of the United Presbyterian Church of Scotland
Signatures of Parties
Rank or Profession, whether Single or Widowed, and Relationship (if any) /Signed/
John Young
25Ironstone Miner
Bachelor
/Signed/
Agnes Twaddle
21her
X Mark
Domestic Servant
(Spinster)
Usual ResidenceJohnGrasswater?
Parish of Auckinleck
AgnesBurnhouse
Sorn?
Name, Surname & Rank or Profession of FatherJohnRobert Young, Putter in Ironstone mine (Deceased)
Elizabeth Young, Maiden Surname Stewart
AgnesJohn Twaddle, General Labourer (Deceased)
Janet Cringan
Minister & Witnesses/Signed/
Matthew Crawford (Minister)
/Signed/
William Young (Witness)
Robert Young (Witness)
So that's that bit.
I didn't have this info in the library or I could have found the Ayrshire census for Robert Young hey ho.
Here's Agnes and John in 1881 in case you don't have it...
Dwelling: Cronberry Crossings
Census Place: Auckinleck, Ayr, Scotland
Source: FHL Film 0203588 GRO Ref Volume 577 EnumDist 10 Page 10
John YOUNG M 40 M Old Cumnock, Ayr
Rel: Head
Occ: Railway Labourer
Agnes YOUNG M 35 F Sanquhar, Dumfries
Rel: Wife
Robert YOUNG 9 M Dalmellington, Ayr
Rel: Son
Occ: Scholar
Agnes YOUNG 6 F Dalmellington, Ayr
Rel: Daur
William YOUNG 4 M Dalmellington, Ayr
Rel: Son
Jane YOUNG 11m F Auchinleck, Ayr
Rel: Daur
John NAIRN U 30 M Glasgow, Lanark
Rel: Lodger
Occ: Wire Fencer
Quintin BROWN U 25 M Dalmellington, Ayr
Rel: Lodger
Occ: Wire Fencer
Some Sanquhar census material that may help.
1841 Sanquar
Lochend
John Cringan 40 Coalminer Y
Agnes Cringan 42 Y
William Cringan 19 coalminer Y
Janet Cringan 16 coalminer Y
John Cringan 6 Y
1861 Sanquar
27
house of Ker family, father is James corn farmerAgnes Twaddell Serv unmar 15 Domestick Serv Sanquhar
There were lots of Russel/Russell families in Sanquhar - Janet's mum may have been a Russel or it may just be a very kind friend/neighbour. I have a Joseph Faulder Fallon in my family the Faulder comes from the name of the gent who employed Joseph's father when he was orphaned.
If you think of anything else you need a look up, give me a shout.
Cheers,
Pam