I don't have both but I know that a lot of medications increase the risk of bipolar. It's not the diabetes, it's the meds. Like Seroquil for example is one. I can see where it would be tough to manage both because changes in blood glucose effect your mood.

As for people who have bipolar already and get diabetes, I work in a diabetes education clinic. I would say in the past four years we've seen only about 4 people with diabetes and bipolar. Depression is very common among people with diabetes, probably about 85% have both.