Many of us know the path you are about to walk. Many of us recall the day we discovered our life partners betrayal. The journey you are on will change you. Not necessarily for the worst so please try not be be afraid. You have had a terrible shock. The joviality they display at your trusting is abhorrent.
The best quote I have read on here, as part of my journey is
'When someone shows you who they are, look'.
Those messages show who they are, and your actions now, will show who you are.
Now is the time to be kind to yourself, self care is going to be key to building up the strength and resilience you are going to need to embark on this journey. For now, concentrate on basics. Food, rest and kindness to yourself. Have a select few friends and/or family you can lean on and when you need to, you can trust they will be there. A support network is invaluable. Your mind will be in a tailspin, it's normal. Lots of posters here will have practical advice. Listen to it, list their 'to do' list. Sadly, it comes from experience.
All the posters will walk this journey (virtually) with you and I certainly found these boards tremendously supportive.
I'm so very sorry you have two snakes in the grass to deal with. But deal with them you must, for your own well-being. This is no longer about him or them, this is for you and your children