I don't see anything in that article to suggest 1 primary caregiver is best. The sense I got was that children got a benefit from having a strong attachment, but no arguement or experiment to test for weather 1 strong attachment is better than 2, possibly weaker, attachments.

Also, I am always suspisous of 2nd hand science reports, especially when they do not provide a direct link to the original paper(s). If anyone does track down some research papers on the subject, please post it here.