What symptoms of double diabetes are the most common?

Metabolic. Polyuria (increased urination) polydipsia (excessive thirst) and polyphagia (increased food intake) are the common issues. After that it depends on the severity of illness and treatments. In severe cases coma and death can ensue.
Same as others. Double diabetes is when a type 1 diabetic becomes obese and develops Insulin resistance at the same time. This person now will need very high dose Insulin to overcome the Insulin resistant currently exist. The symptoms are the same as other diabetes: thirst, urination, tiredness, wounds don't heal, blurry vision...You can also have no symptoms at all if the diabetes is currently controlled.