To answer your question. I cannot say your wife took very seriously her vows, you have shared little about her, but what you have shared is indicative of solid character.
If your wife has solid character I would expect her to take her vows seriously and be faithful.
Truly this is about trust. You have betrayed the trust and faithfulness your wife has vowed to uphold. You made the same vow, but you broke your vow.
So this is not even or reciprocal treatment. She is existing under one standard and you are not meeting that same standard. THAT is the crisis you face in your marriage. It is a crisis of trust.
If woman #2 tells your wife it is much worse for you as noted above by
@bmp2cpm (he is married as well) for the reason that not only did you break the trust, but you perpetuated the lie. Woman #2 knows this perpetuating the lie is the most hurtful thing and why telling your wife is so destructive.
If you tell your wife then your mistress loses the shock and pain of the surprise. It dispels much of the power your mistress wields in telling your wife your big secret.
This is going to deeply hurt your wife. You must accept this & stop trying to avoid it. Your marriage may not recover, but it may if she loves you & will forgive you. But you will be subject to suspicion and distrust from your wife for a very long time even if she will keep you. Trust must be built over time but can be destroyed in an instant.
See this for the betrayal it is and see it as the trust crisis you created. You need to get your priorities straight and figure out what is important long term.
Your wife will be deeply hurt. That is unavoidable. But how she discovers the truth will have an enormous impact on the outcome. And if you tell her you must walk away from the mistress forever & know she will make efforts to destroy you.