I am right where you are LMom - going from HS 1 to HS2. I'm still happily in denial. My 2nd child would be in kindergarten (and she just turned 5 this summer). How bad can I screw up? We are actually leaving on vacation to go the Black Hills on labor day when everyone else is packing their backpacks to rebel.
I will say HS 1, while dealing with the preschooler (although she was attending a very part to PS, which almost felt like more work to get her there hurry up and turn around and get her again), was definitely easier and more fun than I thought. You'll do great!
I am right there with you. DS5 turned 5 in summer as well. He too attended PreK last year. We did most of the homeschooling when DS5 was in PreK and it worked quite well. How old is your older one? Mine just turned 7. I am still getting used to typing DS7

My main worry is how to keep DS5 busy while giving enough attention to DS7 when he needs it. DS7 and I are both used to the one-on-one situation we enjoyed last year. It will be hard to leave it behind. DS5 and DS7 have different personalities. What worked for DS7 may not work for DS5.