Sri Ganapathy in the ashtothra namavali is addressed as Dwai Mathurayai nama:That translates into having two mothers.He was created by Parvathi out of her makarandha powder smeared on Her body before taking a bath.She wanted someone as a guard so that she is not disturbed while taking Her bath.She created an image of a small boy out of the makarandha powder and gave it life and this is how Sri Ganapathy was born.He was to guard His mother.How He got His elephant head is known to all.He is therefore praised as dwimukhaya nama:Having had a human being face first and then the face of the elephant.Ganga Devi is deemed as Sri Shiva's second wife though there is no such thing as Ganga married Sri Shiva.Only that  when  Ganga descended from the Heavens to earth Lord Shiva was prayed to hold her on His Jada mudi and release Her through a strand of His hair by Bhagiratha.
Thus Ganga became a permanent resident of Sri Shiva's head.
As She is always with Sri Shiva She is deemed as His wife thus becoming another Mother to Sri Ganapathy.
Dwaimathurayai Nama;  thus got included in the Ashtotharam.