Hi Nicola,
I think the Consultsant you saw did not explain what she meant.
http://www.everydayhealth.com/type-1-diabetes/double-diabetes.aspx
There is such a thing as double diabetes
Obesity is the main risk factor for double diabetes. Patients with double diabetes are most often overweight. Doctors suspect double diabetes might be partly caused by type 1 diabetics who take insulin, but haven't made the other healthy lifestyle changes needed to help manage the disease. Because insulin use causes weight gain, if the type 1 diabetic patient doesn't take steps to eat healthy food and exercise regularly, weight gain could begin making his or her cells insulin-resistant.
Treating Double Diabetes
Treating a person with double diabetes is complicated because the preferred methods of treating type 1 and type 2 diabetes are different: