This is an excerpt from an ESL reading book. What I want to ask is whether "Their independence and ability" can be regarded as a single concept, so "is" can be followed.
The number of working women has been steadily increasing in recent years. Most significantly, many mothers with young children have been returning to work rather than staying at home. Furthermore, the “glass ceiling” that for so long has kept women from being promoted to executive positions seems to have been shattered. These women, who are able to balance their roles as both mothers and corporate executives, are being referred to as “alpha moms.” Their independence and ability is quickly erasing past stereotypes that portrayed working mothers as overworked and exhausted.