How Can I Wire This Layout Plan

You could wire the one transformer to the outer rail say, and then using a spdt switch, switch the inside rail from second transformer to first transformer.

Or what I would do is run TMCC engines and a control system.

Isolate the loops from one another. Use two transformers. The black or common are connected at the transformers(properly phased) and one common distributed to the layout.

Now, the fun part. You need a double pole double throw Center Off switch. Wire the hot from each transformer to one of the center poles. One end run a wire from one end pole to a loop and run a wire from the pole next to it to the other loop. The switch in that position is for two trains.

Now run two wire from one pole on the other end to the center rail of both loops. The switch in this position will run one train on both loops from one transformer.

My sketchy sketch

Simplest way? A transformer with two handles and just switch when you cross over.

John H posted:

Simplest way? A transformer with two handles and just switch when you cross over.

Excellent! Too obvious. I overlooked it and assumed he had multiple transformers.

I would be concerned about bridging the outputs crossing over.

how about this idea.  Assuming you have transformers properly phased.  Place a SPST switch to connect the inner and outer loop's center rails.  Now connect a 14V bulb across the switch terminals.  With the switch open, the bulb will glow, brightness dependent on how much voltage difference between the loops (dependent on how fast/efficient engines are).  When ready to run both loops with one train, dial one transformer up or down until bulb glow is minimal or out, then close switch and cross train over to other loop.  Open switch if want to restore two loop operation. Slight voltage differences should not affect speed significantly.

Only drawback is bulb might pass enough current to keep an engine moving, affect the reverser unit or keep engine headlight on.

But just an idea. Take it or leave it.