From the summary by Nancy Polikoff:
The NLLFS has been following 85 children of lesbians born through donor insemination beginnning in the 1980's. Information about the study and its earlier published research is all located on the NLLFS website.
Of the 73 two-mother families in the study, 40 couples had split up by the time their child was 17 (over 90% of these occurred before the child was 13). This is a higher rate of separation than the divorce rate for married heterosexuals. 71% of separated couples reported shared custody of the children, a number considerably higher than the rate of shared custody among divorced heterosexuals. 59% of the couples had completed second-parent adoptions, and parents in that group were more likely to share custody. The children whose mothers had completed second parent adoptions were much more likely to report feeling close to both mothers.
Neither adoption nor joint custody showed any correlation with psychological health or problem behavior, but that may be related to the small sample size. Marriage was not part of the study because nearly all the separations happened before gay marriage or civil unions, etc. were available.