When my 8-lb. son was in the NICU, he was there with the 5-lb. crack baby, and the 4-lb. triplets (who had just transferred in from McMaster). 
Of the triplets, 2 of them always shared one isolette, and the other insisted on having his own.  If the two were separated for any reason, they would kick up a huge fuss, and all the monitoring alarms would go off. 
I can't tell you how many times I went in the nursery, only to find the two of them snuggled up together against the chest of one of the nurses, in a rocking chair.
Those triplets live just down the road from me.  The same two are still inseparable.