When one guardian is a part of two families, the guardian will need to be added as a guardian on each family separately and create two separate accounts. The guardian can use the same email address. They will be asked to link accounts so they will be able to use the Switch Accounts function.
For example, if Guardian A has two children with Guardian B and one child with Guardian C, they will need to create two separate accounts. If they create one account, all three children will be listed as siblings and Guardians B and C will see the other's children. Once Guardian A creates two separate accounts from the email sent to him twice, he will see the families separately and will be able to switch accounts between the two.