Wah two under two is no joke lah, you are really a supermum! Maybe you can consider putting the 2 month old with you and husband in the master room since newborns still need night feeds, and let the helper and the 19 month old share the study room together so the toddler's sleep regression won't wake the baby. As the kids grow older and sleep gets more settled then you can slowly transition them to share a room together, but for now just survive first lah, do whatever works best for your family and don't stress too much about the "right" arrangement!
We're doing toddler with dad in another room and baby and mum in primary room. Been working pretty great so far as compared to both kids with mom in one room. The older one will wake or toss making sleep unsafe for baby. And I only wake for baby for easier breastfeeding and diapering. Hope it works out for u