Excellent question. Five years ago, I started my company with my partner. Before that, I joined his first startup. All in all, we've been working together and living together for almost 10 years. Our families are close and we have the same group of friends. Basically, we're around each other all. the. time.
Both our current company and our last one were in the adult space (sex toy design and manufacturing, and our adult mobile app store). For a few years we ran both companies simultaneously, mostly from home in a room that was both our bedroom and office (because 1. #startuplife and 2. adult startups don't usually get funding that affords us office space). We even moved to a small factory town in China for a couple of years to lead a group of designers and engineers and oversee manufacturing, where we had no friends aside from each other.
Sounds maddening, doesn't it? Except it isn't.
Some might say there are a lot of land mines here. Having virtually no separate life from your partner is dangerous, working in the adult space can be hazardous to your sex life, the pressures of running a company can turn you against each other. But only you and your partner know how you tick together. I believe that as long as both of you are putting your all into it (the relationship and the business) AND you genuinely love and admire your partner for who they are, vice and virtue, it can be an incredibly rewarding experience. You'll both be working for the same dream personally and professionally. Remember some of the greatest companies in the world are family-owned.
Things I would look out for are not so much about what your partner does or their personality traits, but more about how you both are together. How do you argue? How do you make up? Do you turn to each other when you're under stress? Do you value each other's input? Do you inspire each other? Can you tell each other when the other is slipping up? Can you handle harsh criticism from the person you're most vulnerable to?
If you haven't already, you need to learn how to argue. Say what you mean, criticize what they do not who they are, and when you're done, apologize right away. Both of you. Actually, both of you should get used to apologizing a lot. I've found that because you're under a lot of stress, the smallest jab can become a war wound. Likewise, offer accolades often too. Unlike traditional business partnerships, you don't get a lot of emotional distance when you work with your significant other. I'm not suggesting you coddle each other either. You need to learn how to give and take criticism with the understanding that it can be emotionally charged and received.
You should inspire each other and feel accountable to each other to work efficiently and effectively. If either of you don't feel this way about the other, I'd advise against working together because inevitably one or the other will feel they're pulling all the weight. You don't get extra leeway because you're sleeping with your business partner.
Speaking of sex, embrace the quickie. Because, let's face it, you're not gonna have a lot of time all the time.
Lastly, and importantly, don't shit with the door open. Also, take a shower, brush your teeth, put on pants. Keep some of the little mystery you have left. :)