Here's a different take on your situation that favors the big boy plus one smaller engine.
The big boy gets a "mainline" with maximum curves and a tunnel, bridge, and grade up and around the back that frame the center of the layout.
The inner loop, which was already tight and small, has been broken up into a series of switches. The big boy can drop cars on the front straight, a smaller engine could pick them up and move them about inside the loop while the big boy runs.
Increasing the curves also created the possibility of putting control equipment on the front right corner.
