What resource should we use for confirmation candidates who are cohabiting?

Refer the couple to the Catechism of the Catholic Church, which says that one must be in a state of grace to receive confirmation (CCC 1310). Unmarried couples living together as husband and wife are not properly disposed for it.

The Catechism describes the aim of confirmation preparation: to lead the Christian toward closer union with Christ and greater familiarity with the Holy Spirit, his actions, gifts and promptings, so as to take up the apostolic responsibilities of Christian life. It should awaken a sense of belonging to the universal Church and to the parish, which bears special responsibility for preparing confirmands (CCC 1309).

The state of grace is not required for confirmation to be valid, but it is required for it to be received lawfully.