If all datasets contain meaningful data (such as after loading of a series from another TotalFit session) you can use update_parameters_from_all_datasets() to send them all at once into the all-parameter vector. NOTE: this method may show unanticipated effect: if the parameter in the two datasets is linked but have different values, the value from the dataset with larger serial number will be propagated to all linked parameters!
For a specific usage see TotalFit.m