Here are some solutions...
This is the concentric, commonly-used 84 track nested in the 96, with approximate 6" track spacing, taking up roughly 108" of layout width:
Here is one-of-many solutions for nesting two 96 curves (back to back) using four (4) 5" straight track sections for the outer curve:
Admittedly, it is not optimal in keeping a consistent spacing between the tracks but it will give you an idea of the strategy to use two 96 Fastracks in concentric curves. Many other solutions are possible.