Hi
When twin births are registered, they either have the same page reference number or a consecutive number, there are 10 birth registrations to a registration page.
I would suggest that they aren't twins. The fact they could have the same birth date may be purely co-incidental. There are 5 Roberts births registered that quarter at Kingston. Roberts is a frequently occuring surname, there are 982 Roberts birth registrations in Sept 1908.
The only way to find out for definite is to purchase both certs, birth registrations before 1911 don't have the mothers maiden name on the index which may have ruled one out.
Dawn