I cannot see how it is possible that step back can in any way match the speed of double revolution?
The press indexes the same speed when it steps back...
When you run the press around 2 times you only print the white and flash on the first revolution right? Then the second revolution you print the white again, flash, and top colors (Or just print if it is a one color).
When you use step back it prints the white, flashes, steps back, prints, flashes, and moves forward. The print is exactly the same, just does it all in one revolution.
If the print is a one color white, and your flash time is longer than it takes to print the white, running around twice may be faster. In this case on the second revolution the flash should be turned off.
Like you said earlier tho, if it is a decent sized one color white run, using 2 screens is by far the best option.