This is a true story - I know most of the people involved:
Two women in a long-term relationship, and two men in a long-term relationship were old friends. The two couples got along great and loved each other. They decided to have a child together, that they would co-parent in their two separate households. One woman wanted to be pregnant, one man wanted to give his sperm. So this is what they did - The women went into one bedroom. Down the hall, the men went into another. The men made love, and collected the semen. The lover of the semen-giver went out into the hall and handed the semen to the lover of the pregnant-to-be woman. She went into the women's bedroom and somehow shot the semen up into her lover. The first month it didn't take. The second month, the two couples became pregnant with a beautiful boy who grew up with two loving sets of parents. He is now about 21 years old.
Yesterday, I was watching, on the Logo channel, comedian Ant's touring show across the U.S., and in the episode on Montana, Ant visited with two male ranchers (lovers) who were living and working with two women (again, lovers together), who were embarking upon precisely the scenario you describe (about having and raising a child together...not necessarily about how the child was to be conceived). It was a little like watching Ennis and Jack living that sweet life, and then some...one of the men's mothers was present, and was extremely supportive and loving towards her son and these important people in his life.