Oh dear, you are really up against it at the moment! Honestly, I'd be tempted to stick them both in front of the telly for a bit while you have a cup of tea and a few minutes to calm down. Can you take them to the park later this morning so they get a bit of exercise and fresh air?
You are at a difficult stage, but I promise you it does get better and easier! Think about what you like doing with the kids. Baking? Crafting? Kicking a ball around? Going for a walk/scooter ride? Meeting up with friends who have children of a similar age? Playing Hide and Seek? It can be difficult to feel inspired when they are both screaming, so write a list while they're watching TV and then you can quickly pick an idea from the list and say, "Right! Let's go and do..."
Try to get them outside every day, even if it is just a walk/scooter ride. Try to avoid them having "individual" screen time (eg one child looking at a phone/tablet alone) if you can but don't feel guilty about a bit of communal screen time eg watching some kids' TV together and chatting about it.
Edited to add, my DDs loved sticker books when they were three, and that's a nice quiet activity to do together while the little one is napping. If you have any magazines or brochures, cut pictures out (animals, furniture, food etc) and stick them onto blank paper together - that's quite fun.
The lack of sleep is awful. Do you have anyone who can share the obernight duties with the baby with you? Is she waking for milk, or for unknown reasons?