Here's my experience in case it helps. Sorry it's a bit long.
I had twins at 28 weeks, and already had a 2.5 year old DS. I spent a couple of months in hospital before the twins were born, and about a month after they were born. DH used to bring DS1 to visit either daily or every other day (I was not always in a local hospital).
DS1 did not appear to be too affected by my absence, until I came home from hospital when the DTs were about a month old, and had been transferred to a local hospital.
Once I was home he started waking in the night and calling out for me, which he would never normally do. This stopped after a couple of weeks. One evening I told him I was going to the hospital to visit the twins and he was extremely upset, because he thought I wouldn't come back (there was a lot of being rushed to hospital suddenly and staying for weeks when I was pregnant).
Because DS1 had clearly been more affected than we'd thought, I decided to spend my days with him, and then go to the hospital to spend the evening with the twins once DS1 had gone to sleep, or go very early in the morning, and come home to get DS1 up (I couldn't sleep anyway...). Most days I would take him to the hospital during the day for a short visit.
It may have been paranoia, but I felt that some of the nurses and doctors were critical of the fact that I was not at the hospital 24/7 with the twins. However, rightly or wrongly, it was my decision to spend my daytimes with DS1, who was aware of where I was, rather than with the twins who had constant medical attention, and didn't know what time I was with them, or how long for.
I felt extremely guilty and still do, but would do the same thing again. DS1 is happy and shows no signs of resentment towards the DTs.
The DTs are now 6 months old, and even now, if I have to take one to a hospital, DS1 becomes very unsettled, although he doesn't say anything - just stops using the potty, for example. I'm careful to always tell him when I'm coming back, which seems to help.
On a practical level, some hospitals have play specialists, who will spend a couple of hours with siblings whilst parents spend time with babies, speak to the doctors etc.
Hope all goes well with your DTs - it's a worrying time, but it helped me to think that NICU/SCBU was just a small part of their life.