The manual way would be to create a dummy corridor that samples the existing ground and then run the report for both corridors and then combine the reports in Excel to get one report. If you let me in on what version of Civil 3D you are using I might make a blog post on how to change the report to have the slope stake report provide the info you want directly, since this report may come in handy if I ever do another airfield project.
The SincPac might have something that would work also, but I'm not sure.
Christopher
Civil Reminders
http://blog.civil3dreminders.com/
http://www.CivilReminders.com/
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