1. LTM8024 channel 1 can deliver enough power to serve as input to channel 2 as well. I would check efficiency+thermals for both scenarios before deciding on teh preferred setup.
2. With VIN2=Vout you can use higher frequencies as step down ratio/Duytcycle is not at the edge. For example 3MHz are not doable for 12V down to 1,2V. But 3MHz are doable for 12V down to 3,3V and 3,3V down to 1,2V. It's your choice depending on other constraints (efficiency, thermals, externals,...) You should start with the datasheet examples. There will be a reasonable large frequency range left which is doable from timing and general operation. I think 1MHz is a good starting point.
3. Startup with additional capacitance will be delayed, especially as LTM8024 offers frequency foldback in overcurrent/startup operation.But I do not expect any general issues.
LTSpice will be of great help to get more accurate answers to my statements (on all your questions)
Thank you for your fast asnwers and suggestions on starting values! I will definitly simulate it more in LTspice.
My biggest concern was that 3.3V was to low for VIN2, even though it is stated that when VIN1 is above 3V, it is sufficient if VIN2 is above 2V.
Regarding capacitance, delay due to foldabck is not an issue, it is important that it is not stuck due to some protection.