A blue red composite course would easily be ranked in the Top 10-15 modern.
Can you tell us the holes/routing for this composite course?
Blue #1 - #6 and Red #7 - #18
While I don't care to speculate about how a non-existent composite course would be ranked, if we are going to do a composite routing exercise at Streamsong my pick would go something like:
Red #7-9
Blue #17-18
Blue #1-6
Blue #8
Blue #12
Red #14-18
This does introduce a relatively long walk from Red #9 green to Blue #17 tee, but otherwise I think it achieves a good balance, including most of the best holes from each course. It's even a relatively compact, contiguous section of the property.
With this routing you could also swap out Red #8 for Blue #7 depending on how you're feeling on a given day.