LBC, that you didn't freak out at the suggestion says so much about your character already!
Me personally, I wouldn't be able to handle it. I'm not very good at sharing and I know this. I can be the third wheel in a threesome, but I can't be one of the main two people in the couple part (provided all three aren't separate wheels, of course).
That being said, if you are seriously considering this as an option, please allow me to suggest three things:
1.--Establish rules from the get-go. These rules should reflect what is and is not okay in your marriage. You should sit down one night, fully clothed, fully sober, and just talk about it. Explore what you think you'd be okay with and what you know you definitely would not be okay with. If you're on the fence about anything, I'd put it in the "Not Okay" category to begin with. You can always try new things, but you can't undo anything you've already done. I also suggest writing them out, but that's just me.
2.--I'd suggest that one of the above rules is that you're both present anytime there is sharing. No solo sex with other people unless you're going to go with the complete open marriage thing. (You don't seem interested in that.) I think establishing that you both must be present (but not necessarily involved) in any play further solidifies that this is just that: play. This way, there are no secrets and the third person can only be involved in your marriage as much as each partner agrees to together.
3.--Do NOT choose a past lover for these activities. There's history there that you aren't a part of. Leave the past in the past. Find someone that you both feel is safe and attractive and okay with your agreement. But turning to old lovers will bite you in the ass eventually. Don't go there. Don't go back--always forward. Pick someone with no history, no emotional connection to either of you, someone that--should it not work out--you don't ever have to speak to or see again.
And just maybe a little suggestion? I'm not sure if this is even possible, but how about using a strap on with her or perhaps just a hand-held dildo while you're fucking her ass? Then you get the benefit of a hot DP without the additional body attached.