1. Be really damn sure you want it.

2. Hopefully as a married couple you trust each other enough to bring up fantasies. Mention it to her next time you're talking dirty; you can also build up to it, maybe just talk about having her flirt with another guy first while you watch (maybe a stranger, while you sit a bit off so he doesn't know you're a pair). Actually, going to a club and having her flirt, kiss and grind with another guy might be a better way of checking 1. without going too far.

3. Let us know how it turns out!