Your chances at co-guardianship will depend on whether this guardianship is temporary, interim or permanent. If you both have permanent guardianship and the issue of termination of parental rights is not in the picture (i.e., so that you both can adopt the nieces), you will need to petition the court to ensure your guardianship remains.  It is a legitimate concern because the court needs to do what is in the best interests of the children, so divorce and guardianship issues will need to be considered in light of what is most advantageous to the children.  Your guardianship is not going to unilaterally terminate but may be impacted by what your spouse attempts to petition. You should go over all of this with your family law attorney to ensure your specific situation is fully reviewed.