(((twinmam))) you poor love, its a nightmare situation to be in and I'm just glad you've spilt a bit somewhere. Holding it all together is too much to bear, even though you are having to bear it for most of the time.
I was in v similar situation for a week when dd2 was a month old, and dd1 was just 3 (it was her birthday, and Xmas, when dd2 went in to hospital). Reading your post brings it all back, and I can tell you honestly that you life WILL return to normal, and at the same time, that I have not forgotten what it felt like and it hurt for a long time - not least that feeling of being torn between my LOs. dd2 recovered fastest, and dd1 needed to talk about how she felt when dd2 and Mummy were 'in hospital' for some time - but is fine now.
dd2 has a floppy larynx, and ended up in HDU on CPAP for a week with bronchiolitis as a result - we've had ENT issues with her since, but never anything as scary, and she's very much a normal 2.5 yr old now.
Its utterly surreal, trying to go between the intensity of hospital life and attempting normality with your other LO for the brief periods you get home. Surreal and very, very exhausting.
Adrenalin does get you through, as does the kindness of strangers (our family were all away, it was just me and dh), until you can let go a bit...and IME, thats when the exhaustion and grief/anger kick in. Its the start of getting back to normal.
Meantime, the things that helped me: talking to others who'd been through similar and were through it; chocolate without guilt; the hospital's parents' room; shouting and crying in the car between hospital and home; reminding myself that there was NOTHING I could do to make it all better for my dds, except what I was doing, and that they would be ok and recover after the crisis had passed. And they are.
Will be thinking of you lots, let us know how she is, and how her sister is, and how you are especially, when you get a chance.
Most of all, wishing you little dd better and out of IC asap xxx