With LO#1 we used CIO to get him to sleep, but he was already having longer stretches, sleeping in a bassinet

LO#2 will be 4months in a few weeks and he's still hardly sleeping, he goes to sleep ok and sleeps in the RNP then wakes up and wants to nurse every hour and will only sleep on me

Their rooms are across from each other.

I want to transition him to his crib and I'm ok with CIO with checks or modifying it dependent on LO, but I'm really concerned with disrupting LO#1's sleep. Once he's asleep he sleeps through everything but I can't handle two non sleeping children.

Did you CIO with two children? What did you use to help?