Random thoughts, in no particular order, with likely-varying degrees of helpfulness
1) I worry about your exhaustion of handing a full-time career plus keeping up with classical homeschooling in the mornings. I work very part-time one evening/some weekends and am the primary M-F/9-5 caretaker and it feels like just enough on my plate.
2) What will the kids do after 12:00? Will they be under the care of someone who will take them to homeschool get-togethers so they can maintain friendships and out-of-the-house activities?
3) Half days is totally enough to "school" two kids with a 1:2 ratio. It's amazing how much we cover in an hour over here.
Edit to add...
4) Curriculum is as cheap or as expensive as you make it. Boxed packages probably won't fit for a gifted kid with likely varying levels of abilities in different subjects, so I'd look at individual curriculum for whatever subjects you want to focus on, personally. There are loads of free resources though between inter-library loans and the internet, so don't let cost deter you.
Homeschooling in our house means we focus on the Big Three (reading/writing/'rithmetic), and let the rest be interest-led (lots of ancient history, early American pioneer times, and nature study around here).
