I will pray for the survival of the one twin, if that be God's will, and for every spiritual assistance for the mom and dad. Dear Lord, in Your mercy, embrace and help them. Gather us one day all together in Your kingdom.
I think this is the best kept secret in the Catholic Church -- a Catholic centered weekend for grief recovery. It is open to all faiths, but ends with a Mass. You can check the link for a presenting team in your area.
I do not personally know as a mother the sorrow of miscarriage, but I have seen two of my sisters and one sister-in-law suffer through them. God blessed me and my other sisters with trouble-free pregnancies. Hugs to you.
Thank you for your prayers.