Although it's not my style, I would go with something similarly trendy to Madison but I would steer clear of names ending with "son". This only would further emphasize that you named your daughter a male name/surname.
DD1 is Addison and had this one been a boy she would have been named Joshua. I think Madison and Joshua sounds great together and I hadn't seen it listed yet.
Re: need a brother name for DD Madison
Not sure of the exact style you are going for, but maybe another top 25ish name for your son?
Aidan/ Aiden
Liam
Jackson
Noah
Ethan
Logan
Gabriel
BFP 1- EDD 2/09/11 Missed MC DX @11 weeks D&C- 7/25/10 BFP 2- EDD 12/22/11 Natural MC @ 5w 2d BFP 3- EDD 1/25/12 DD Josephine born 1/16/12
Evan
Mitchell
Jacob
Christopher
Although it's not my style, I would go with something similarly trendy to Madison but I would steer clear of names ending with "son". This only would further emphasize that you named your daughter a male name/surname.
I know an Addison with