Karen, I am basically an introvert but have been remarkably successful in interviews. This coming from someone who used to throw up before being able to drive a car down the street, so no stranger to nerves! My secret, if there is one, is
a: to be prepared, a lot of interviews now are trying to test your previous behaviours, so they will usually ask you to tell them about a time where you have solved a problem/came up with a new initiative/dealt with a difficult situation etc. the key here is to not have millions of scenarios prepared - have 4 or 5 that you can adapt depending on the situation/question. Think of some aspects of your work that you want to get into the interview and use them as examples - make them fit the question!!! You can do this even in a more traditional interview by just quoting examples in response to questions.
and b: Visualise yourself in the interview, see yourself handling the questions well, see yourself being calm, personable and wowing the interviewers with your articulate answers. When you go into the interview, be the person in your visualisation. it's kind of like putting on an act that protects your "real self" (the nervous wreck!!) from having to deal with the questions, while Super Karen does all the hard work. See them liking you and offering you the job.
I know it sounds mad, but believe me it works. The last interview I had where I failed to get the job was in 1982 and I was still at school.
As far as being up against your OH goes, I think it would help if you sat down and prepared together for the interviews and rationalise it that you are a couple, if one gets a promotion and the other gets slightly downgraded, then as a couple you will be in roughly the same position. Therefore, it doesn't matter which of you gets the upgraded job as long as one of you does
Hope it all goes well for you both - just keep seeing that success in your head, the p