my poor little brain is already at breaking point
So, if we remove the connection between the two circles of track..
Left-line running, so the outer circle runs clockwise and the inner circle anticlockwise..
We are DC, so let's assume 'left-rail positive :
The outer rail of the outer loop will be' red' (positive), but the inner rail of the inner loop will be 'red'..
If we then add track between the two loops, we end up with a short circuit, BUT if you use separate power supplies for each loop, you can 'sort-of' connect them together, but not easily run a train from one loop to the other.
You need each loop to be able to be continuously powered, but independently of the other..
But,
You also need a section of each loop, plus the bit joining the two loops, to be able to be fed power from either loop.
Depending how you pick up power (assume just loco and tender) the perceived wisdom is to have 'the bit in the middle' long enough to hold the complete train.
So to cross from the outer to the inner loop.
Central section is connected to the outer loop, but disconnected from the inner loop.
Drive train onto central section, and stop.
Disconnect central section from the outer loop.
Connect power from inner loop to central section, and drive train onto inner loop.
PhilP.