It's definitely possible (and relatively simple!) for two moms to nurse a baby, but not necessarily in the way you might think. I am going to respond to this post both as an IBCLC and as a lesbian mom who nursed a baby as his non-gestational mother.
[My response as an IBCLC]
Although you are currently lactating, you will find it is quite difficult to significantly increase your milk supply at this point in the lactation process. In fact, if that were your main goal—for the non-gestational parent to be the primary milk supply—you'd really be better off weaning your current nursling completely and inducing lactation from scratch (there is a hormonal induction protocol). That is, of course, an option, but it's not what I would recommend unless there was a reason why the gestational parent was unable to produce a full milk supply. The fact that you are already an experienced nurser and are making some milk will certainly be to your advantage when establishing a nursing relationship with your new baby, but the quantity of milk that you are able to produce likely won't make much of an impact, nutritionally speaking. Assuming (from what you wrote) that your partner will be working outside the home, and you will be responsible for the majority of the weekday feeds, the simplest way for you to both achieve a successful nursing relationship with the baby will be for you to nurse the baby using your partner's expressed milk in an at-breast supplementer. (I recommend the Lact-Aid, from www.lact-aid.com .) Your partner would nurse the baby whenever she was home, and would pump while at work.
[My response as a queer NGP who nursed a baby]
In my case, I was still nursing a 2.5-year-old (whom I had birthed) when my then-wife (we're now divorced) gave birth to our fourth son, Leo. I nursed Leo soon after his birth, but in the early weeks I would do it strictly for comfort, directly after my wife had nursed him (and after I had nursed our toddler), so that his need for milk was low, and my production was equally low. I was the primary caregiver for Leo after his other mom went back to work, but since she worked from home, there was never any need for her to pump her milk. She would nurse Leo every 2-3 hours. I would nurse him in-between, whenever he wanted to, which was mostly only when he was tired. I would often nurse him to sleep in the sling. I never considered my nursing sessions with Leo to be providing a significant amount of nutrition, but it was enough milk that it could hold him over a bit if we needed to stretch out the time between his nursing sessions with his other mom. Because his other mom and I spilt up right around Leo's first birthday, I don't know how things would have looked in the toddler years, with two moms nursing. As it was, I nursed Leo when he was with me (during the day, mostly), and his other mom nursed him when he was with her (mostly overnight). Leo was my fourth nursling (I gave birth to my first three), and once we reached the toddler years, I really didn't notice any difference in my experience of nursing him vs. nursing the others, even though I'm certain that I had WAY less milk for Leo. He nursed frequently, day and night, until he was about 2.5, and then we cut back and nursed less frequently (only during the day). Ultimately, his other mom and I both decided to wean him, right around his third birthday.
For me, what mattered most was that I be able to "mother through breastfeeding" the way I had with my other babies. And that was entirely possible, even without a significant quantity of milk. I loved being able to nurse him to sleep, or instantly soothe him with a boob. I still got all the flood of good nursing hormones every time I brought him to breast, even though milk didn't flood out the way it had with my first babies. If actual milk production is what matters to you, then you should consider hormonal induction methods. It's unusual to bring in a full supply, but you could contribute a significant amount of milk. In that case, you would also need to be pumping. Here's a great blog that describes the process of both parents nursing their baby after the non-gestational parent hormonally induced lactation: http://firsttimesecondtime.com/category/inducing-lactation/
Congratulations on your growing family!