I believe Shadow the second son says that at the beginning of your post combat main phase there is another beginning phase. So wouldn't that happen immediately before the post combat main phase
Unfortunately no. The Shadow's triggered ability triggers at the *beginning* of the postcombat main phase, and will give you the extra beginning phase after that phase. So the timeline goes Combat Phase > Postcombat Main Phase > Extra Beginning Phase > End Phase.
I believe Shadow the second son says that at the beginning of your post combat main phase there is another beginning phase. So wouldn't that happen immediately before the post combat main phase
Unfortunately no. The Shadow's triggered ability triggers at the *beginning* of the postcombat main phase, and will give you the extra beginning phase after that phase. So the timeline goes Combat Phase > Postcombat Main Phase > Extra Beginning Phase > End Phase.