I have two sons, ages 33 and 37. They love each other dearly, are tremendously close in many ways and are still competitive and judgmental of each other. And they still "recruit" me to agree about their criticism of each other.
I still get the same reaction you are describing - that somehow I have failed because they are so critical of one another.
When I take a step back, I realize that they were not lacking in love (nor are they now), even though I was a working mom. They are good men, productive, successful professionals and people, ethical, bright, funny and loving.
So being rivals hasn't hurt them.
What's very funny is that they criticize each other for exactly the same trait - talking so much about what troubles them that they forget to share what is working well for them.
My advice - don't work so hard to make it just right for them. Your reaction reinforces their demanding more of you at that moment. Remember they're wrong if they suggest you're failing them in some way - you are a loving, giving mom. Take deep breaths (I mean that literally) and try to reduce your reactivity to the game they both play. You're not damaging their little psyches!
A little about me
Married 42 years, about to become a grandmother for the first time and crazy about both my sons.