Jenthorn, I found those ages the most challenging when I had to HS an older sibling. Hang in there, this too shall pass, and all too soon. It helps to remember that some day, you'll suddenly realise they're grown and it's got easier.
Two things I found useful- one was to write down a list of activities to keep the little ones busy in school time. I posted it up so I could see it. When it was noisy I couldn't even think straight.
Then I would pack a little chest full of activities each evening, a lot sealed in ziplocks. (we made it a big deal, and let our little one decorate the wooden chest and paint her name on it and let her choose stamps it with paint stamps to make it more special) Realising that a 2 yo does not have a long attention span, I'd have several things lined up in the chest.
I tried not to repeat activities too often unless it was a winner- like my one child loved to past old fashioned runny glue all over paper with a glue stick. It was a real mess and a waste, but it "bought" us plenty of time each day. This same child painted the driveway and the deck with water and a paint brush singing to herself or played with maize (dried corn off the cob) in a basin on a piece of canvas for long periods, just pouring and scooping with spoons and little cups etc. I only allowed paint once a week because set up and clean up time took too long.
I also hid some containers of toys and hauled them out at needy moments. It helps to rotate toys and not repeat them too often.
I got a lot of ideas on this site: http://www.redshift.com/~bonajo/preschool_activities.htm
And lastly, Anne, from Anne's Homeschool Place has a lovely book called Foundations about habit training and preparing the preschooler to obey and behave calmly during school time, which helped me a lot. Actually I wish I had it earlier. It was gentle, and full of good advice.