I know having 2 sister wives is an ideal for you. But when my ex h and I opened our relationship, and he fell in love with another woman, he suggested to me she move in and become his 2nd wife, help me with the kids and housework, babysit so he and I could go out on a date, yada yada.
I was horrified! I barely knew this woman and in no way did I want to share living space with her, or let her start to develop a relationship with MY children, just b/c my h had a thing for her, emotionally and sexually.
I know now they were both head over heels in NRE. I made them stop having sex, but allowed them to remain friends. I think you should rethink your ideal of 2 wives. Why would you want to move another woman into your happy home after just a few months of crushing on her? I mean, sure, as a dream it's fine, but most people don't move in together just a few months after knowing each other. It's really best to wait a year or so to see how the dating thing goes, and how much you really get along once the first surge of infatuation has passed.
I do sympathize with you on your differing sexual needs from your wife. If she is completely off sex because of nausea and fatige, and of course, gets touched out by the end of the day from your rambunctious 3 year old, perhaps you can do more housework and cooking to give her time to rest. If you keep that up, maybe by her middle trimester her sex drive will perk up again for a few months, until she gets huge.
I understand how your frustrated sexual desire is now being channeled back into A.