Well it could be.
It's a bit of a long game, trying to keep them from talking about you as you woo the both of them whilst getting them to trust and love eachother as much as they love you. The cards are all there, they've a close working relationship, they've known eachother for some time, they've lived with eachother, fought with eachother, supported eachother. It's an issue of the MC bringing them closer, or tearing them apart.
We could've come right out of the gate with 'I'm wanting an open relationship with other witches' with a hard and blunt right in the face reaction. Because then you know from the beginning if they'd be up to it.
It could be an option now, with a bit of like, irish charm, a winning smile, and making an emotional connection first, sitting down with Minna and telling her how close you and Trude had gotten while you were getting close with her. Your heart is torn because you really dont want to hurt the both of them by just shooting down the other. This isnt a have your cake and eat it too kinda of thing, you're not just something i could think of like that you're a person. We're both adults, stupid, crazy adults capible of our own decisions. It's a lot to ask Minna, and its a lot to put on you. But, Minna, could you, and me, and trude, (And Cupcakski, and Shirley, and Erica, and Ect.) be together? It's a crazy thing to consider, but on this base, with everyone so close, relying on eachother, s love such a hard thing to leap to?
Then you could give Minna a day or two to think it over, let her stew on it. Is she willing to tear you away from Trude, is she wanting to harm the fragile emotion state of the castle? She may have already considered these facts, and provided we dont come off of it as wanting more Sex rather then more waifus/imean Love.