My daughter was twenty months old when we began raising my niece who was weeks shy of three.
Oh boy, what a transition. Two only-child toddlers, one of whom was dealing with trauma and loss. Hah, hah, hah, we've only just come out the other side (one year later)! But they're great now (much happier, stronger, more confident, more connected, more loved, more loving, better balanced, etc.).
IMHO, it's important that kids learn that their needs are no more or less important than than other people's. I think it's possible to do that no matter the number of kid/s a person has, but is often a natural consequence of having sibling groups (generalization, not an absolute).
My kids have Daddy/Mommy dates where they get to have a special activity with each of us. That way, they do get their special time with us. That can look like one going to the zoo and the other for a hike, one going to a movie and the other to the beach, or one reading a book while the other helps make dinner.
They also teach, help and entertain each other, which is really neat. My daughter was much more socially and physically courageous than my niece. She got to teach and comfort my niece. My niece saw my daughter climb the ladder at the park, for example, so she (after some time and encouragement) learned to climb the ladder. My daughter would sit in my girlfriend's lap and give hugs, and eventually my niece learned that too.
Now it's sometimes reversed. My niece has lost her fear of water and my daughter is learning to "go under" by example. My niece loves giving goodbye hugs and to play dragon with her friends, so my daughter has learned to role play along side her.
My niece also "does letters" with my daughter and will walk her through a book, the alphabet, or numbers. They get A LOT of special one on one time with each other. Their gifts are different, and they gain from sharing their gifts with the other.
Right now, my daughter is napping and my niece is outside throwing a stick for our dog. Once I'm finished with this post, we'll go draw and hop on our chalk alphabet. One on one time, voila!
They also learn that neither get's their way, all the time. One night, one of them will get to read two of their favorite books, and the next the other does. sometimes they get to each wear their favorite dress, or eat their favorite food, other times neither/both/the other does. We get to learn to share, get excited for someone else's happiness, and to negotiate our upsets around each other.
Children do not need 100% attention to develop well and normally. Some would actually argue that doing so, can actually hinder their progress.