It's not a tricky situation.
It's a commonplace situation, and there is really only one answer. You already know this, and you already know what the answer is - so I'm hoping you came here because you want back-up for what you already know to be true.
Stop. the. lies.
Stop lying to these men. Just stop. There is no happy future for a relationship that requires lies to function. The OM thinks you're single, your BH thinks you're faithful, you're holding all the cards and creating a false world around both of them. Does that sound like a "tricky" dilemma? Does that sound like it is a situation with no clear answer? Come on.
1. Tell your husband the truth, immediately. If his moral compass tells him to leave you, LET HIM. He's a grown man - a good one, from what you say - and he deserves the right to choose. He deserves - after all the years of being the stable guy you would "run to" - to be able to run to safety himself. It will be hard. Actually it will suck horribly. It is also the most important thing you can do. For yourself as well as him.
2. Tell the OM the truth and then STOP SEEING HIM. Stop seeing him until you're divorced. He's in love with a separated, divorcing woman. You aren't that woman. You are a married woman who is lying to him. He will need to digest that and get to know the real you. If he still loves you, then when you're single you can date him. That's how it works.
3. Stop thinking about what will work out best for you. You are not the only one who gets to choose. You are part of a couple, 50% of the couple equation. 50% of the parenting equation. You don't get to choose a fake story for your husband to live in. He's a super hard worker and awesome daddy, as you said, so give the man the most basic level of humanity due to us all by allowing him to know the world he's living in and make his own choices. At this point you are the only one making choices, and they are all about you.
Also, needing someone to financially support you and having that figure into whether you lie or tell the truth to the two men is really awful. You know that. Erase that from your list of justifications immediately.
You already know all this on some level. So those 2x4's are I think, I hope, what you're here looking for.
It sucks, lilbug, I won't lie. Showing someone who you really are, especially when "who you are" feels like crap, because "who you are" happens to be cheating and lying right now - is scary and humiliating. I was right there, showing my FBH "who I am" and that was a shady, shifty, boundary-less person who wasn't very loveable or worthy of respect. I wasn't a good wife. He sat there and stared at the REAL me and it was terrifying.
But that's how you GET to be the person a man can really love deeply. You show people who you are and you get to work on the parts that are hurtful and ugly, you strive for the parts that are good and pure and loving and each day you get to claim more of the good parts and less of the bad ones. But you have to start by showing your face.
So, yeah. Show your cards to both men. Take care of your marriage first in whatever way that happens. Give your husband the human gift of dignity. Leave the OM out of it. If your marriage is over, start over with the OM with his full knowledge of what he's choosing. Work on yourself to deserve the person you're with.
[This message edited by circe at 3:25 PM, December 4th (Wednesday)]