It sounds like you are wanting someone to join you and your partner/spouse in a threesome rather than having an open relationship/marriage which is what my wife and I have, but we do have two different sets of what we call couples with benefits and that sometimes means a threesome with just either one of them or one of us, although normally it is basically a partner swap.
Our main rules are safe sex and no anal. If you are a heterosexual couple you have to decide if you're having MMF or FFM how intimate you want to be.
Is it a MMF or a FFM you're both wanting, try both, we have.
Male/female penetration is a given as is M/F oral sex but you need to discuss in advance if M/M cock sucking or F/F pussy licking is what you want.
Also could just one of you two and the other person just have sex with each other with the other either not there or watching. I have watched my wife fuck both men and women and it is incredibly erotic.
You need to discuss jealousy. Neither one of us has ever felt even the smallest pang of this, we both have our own FWBs and spend nights/afternoons alone with them and this isn't an issue, but it's important to be honest.
Will this just be sex or will the person also sleep, in the literal sense, with you both that night. We did have a rule of not in the marital bed but that no longer applies when we swap with the other couple.
Main rule, be honest, don't be jealous and don't be afraid to say no.
Go for it, if it's what you BOTH want, it will enrich your sex life greatly.