S.H.
I am not an expert on this, but I think it would be appropriate for you to ask them how they would feel about it. It makes sense to me for one couple to be godparents to ALL the children in one family, but again, this is something you should discuss with the couple. If they do not feel they can do it, they will say so. It might be that they will be very honored by your wanting them to be godparents to both children.