Have you read anything like the Ethical Slut? It's a bit American, but it discusses different configurations of relationships, and how to discuss things, defining your own limits and that sort of thing. There are other books on the same subject, but I can't remember authors or titles just now, and if I go to check, I'll just go to bed instead, given the time of night. 
You do really need to talk about it - "open" can have many different meanings, and you need to know you both have the same understanding. Also, you need to be able to renegotiate in the future - people change as times go on and you need to consider how you may handle it if one of you wants to be monogamous in the future - or if one of your other relationships gets more serious or is going to cross the boundaries you've set for yourselves. Things won't be the same forever, and you need to expect that and be able to discuss it and work out how to handle it. There might be some things you think would be great, but in practice turn out not to work for you.
Do you want to see others only as one-offs or more steady? Does it have to be someone the other person has met and needs to approve of? Heterosexual others only, or same-sex, too? Do you need to give notice, or can it be spontaneous from a night out? Can they come back to your house, or must any other people be off-site? If they come home, can it be in your own bed, or in the spare room? Can you stay out overnight, or must you always return home? Can others stay over? Can the other one veto any other partner?
What about contraception (barrier), STI testing (regular), are there some acts you'll only do together but not with others - or if there are acts you don't both want to do, is it okay to look elsewhere for them?
What about timing? You need to make sure there's time to spend together without others, and you also need to make sure you both get a fair share of whatever you agree to. So you might have one night a week each for others, one night out with each other, I don't know - you also have to allow time for work, get-togethers with family & friends, household chores, and if you want to make time for any hobbies or evening classes or the gym or whatever - there are only 24 hours in the day, and there's a limit to what you can fit in. And what about family? Will you tell? If so, and they react badly, how will you handle that? I'm assuming you don't have children, but if you do, that will add a whole extra pile of complication.
It's not all fine and dandy - any human relationship will go through difficult times, and poly relationships, as they have more people involved will naturally have more obstacles to overcome than a monogamous one, and there are enough threads here to show even they don't go smoothly. But because of that, and because you have to make more effort to make it work, it can be more trusting because it's all discussed up-front, whereas I think some (not all) monogamous couples fall into it by default without really considering if they have the same relationship expectations as each other.
I've been single for about a million years, so I might also be talking a load of bollocks...