def _add_units(units1, units2):
    """Helper method to handle units when quantities are multiplied
    
    Parameters
    ----------
        units1 : dict
            Units to add
        units2 : dict
            Units to add
    Returns
    -------
        units_out : dict
            Added units
    """
    out_dict = {}
    for key in units1.keys():
        out_dict[key] = units1[key] + units2[key]
    return out_dict