A Newborn and a Toddler
My children are three years apart. I found it quite easy to start with the baby up high in a ring sling, in an easy breastfeeding position on my front. Once, the baby was adjusted, I put another sling across the opposite shoulder, then loaded my toddler on my hip. I especially liked that they could watch one another.
Once you have two larger children, you need to consider balance. You probably will want the children on opposite sides, either front and back or side and side. With a baby carrier that allows for back carrying, you can carry a baby on your front and one on your back. It also works well to carry the older child on the back in a Mei Tai with an infant or younger child in the front in a wrap.
If you have twins, the answer is easy: Moby Wrap. You can easily carry newborn twins side by side in the same carrier. As they grow, you will need two carriers, which you can carry front and back or one on each hip. Moby Wrap as a twin baby carrier is an essential gift for parents of twins.