Hooray if it helps you, but there are two things that would make me hesitate before recommending 'double Dutch' (condoms + diaphragm) to anyone.
In order of importance, spermicide is nasty stuff (the most common chemical used was originally invented as an industrial cleaner!) Some people are very sensitive to it, but it doesn't do anyone's skin much good, and this means it can actually increase the chance of getting an infection. It also tastes foul, so if you're receiving oral, you need to make sure there's none anywhere near their tongue.
Secondly, it's possible for the man to feel it's there. Some may not be too keen about that, as it can be not entirely pleasant (especially if the metal spring is involved).
You could ask about cervical caps which are smaller and don't require spermicide. They're not always easy to get though.