she is clearly a different person from Marjorie of Carrick and they have different parents, so it makes no sense to reject Martha on basis of Marjorie already being accounted for elsewhere.
so the real question becomes, when exactly did he marry Constance and therefore which of the two women is mother of his children, or which mother for each of the children?
http://www.douglashistory.co.uk/history/williamlonglegdouglas.htm
This might be helpful it lists the 2wives & children relating it also mentions another daughter of mother unknown?? Have a look through see what you think & ill keep researching in the meantime & see if I can find anything more substantially evident.