YANBU, when my first 2 were small I recall counting the hours until bedtime, starting at 2pm. Give one attention and you can be certain that the other one will scream and/or injure self. Try to do anything else, like cook a meal and all hell breaks loose. I think it's probably normal to be tired from the first week of first pregnancy until the last child is in secondary school.
Sounds like the teatime -tired kids routine is the worst bit, isn't it always. Can I suggest food that you just stick in the oven to reheat/ minimal prep and a judicious use of CBBC? Or stick dinner in the oven and retire to sofa with story and both kids?
You don't say how old DC2 is. My teenage son, when left in charge of baby sister at teatime discovered the combination of strapping sis into high chair, giving her finger food and putting in the night garden on i-player on a laptop just out of reach enabled him to get the dinner on. Genius boy. I had been trying to cook with a hungry crawler howling on my legs and getting frazzled, which of course meant everything took twice as long as otherwise.....He analysed the problem and found a solution, because he wasn't as emotionally wound up about it.
Before that, baby got put in a carseat hung at eye level in the kitchen.
If your DS is too tired for the bedtime routine try bringing it forward half an hour? (Risk of him waking at sparrowfart o'clock, but it doesn't always follow) Or promise him extra time with you if he lets you get baby off to sleep?
Whatever gets you through the day....I think we've all had the walking out the door fantasy.... in my case their dad got there first!