I don't think it is possible....
If you add a ldo on the 12V to 5V, you will have a drop volatge of 7V iin your LDO, with a current of 2A => 14W !!!!
You could have 2 buck, one on the 12V, and the other on the 5V, and separate your 3.3V in 2 disconnected rails of power suplly.
You could try to connect the 2 rails, but there will be some oscillation between the 2 buck.
There are some components which switch between 2 power sources (ie battery, and line). I don't know if theses components could work in the transition region...
good luck