An open marriage might be the thing for you, but I agree with kaliope....there is no way to know how the other woman will react, and if emotions get involved, it quite easily could destroy your marriage. Talking to her about it might be helpful...if she won't consent or objects strenuously to any change, possibly marriage counselling could be considered instead.

In any case, I do sympathize with you and your situation.....Hope things get better quickly.