I'm going to assume you mean the "Adventure Paths." If so, I'd say running two in a row with the exact same characters is not a great idea. The paths tend to be very far apart from each other in multiple ways, from geography to theme. You can't just lower experience gains, as you would need to change all the later encounters to be suitable for lower level PCs. Further, when you start the next path, you will have to adjust everything up. In many cases, the second path's adventures will be inappropriate for high level characters, such as powerful PCs fighting goblins, or making non-combat encounters trivial with high level skills, abilities, and magic.
In the end, doing something like this would be far more work than it would be worth. Is being able to run the same characters through two paths a major issue for you? Most people report it taking at least a year to get through a path, at which point many are ready for a change. If not, there are articles in the final volume of each path giving ideas on how to continue that particular campaign, should you so wish. Cramming two unrelated stories into one probably isn't going to give you the results you would hope for.
