Hi there, yes they can be confused. There is much information on the internet or books about the two illnesses but without going into the science of it......

1. Bipolar mood swings are more "marked" or "pronounced" than BPD mood swings. They also follow more of a pattern. There are also different types of bipolar

2. Bipolar is said to be "chemical/genetic" with environmental and chemical triggers and BPD is said to be predominently trauma based with unpredictable mood swings and identity disorder(i am severely oversimplifying things).

3. Some bipolar medication( eg: lithium) can be used with effectiveness on BPD patients to help stabilise the mood swings.

I was diagnosed BPD 15 years ago and dx Bipolar type II in 2007. So I guess you can have both. It is definitely worth exploring with your husbands therapist(a psychiatrist worth their salt will asses and observe his symptoms over a period of time before making a dx). Successful treatment can make the world of difference in learning to love yourself with these illnesses and lead a relatively fulfilling life.

Good luck. Let us know how you go