A solution is described in details in the article PowerPoint 2010: Perfecting the Motion Path.
The basic idea is that it is much easier to look at the problem upside-down.
If the slides are S1 and S2, start with S2 and then create the matching motion path in S1 that ends in the starting-point of S2. See the article for more details, including screenshots.
A comment following the above article recommends using the free Motion Path End Position add-in to PowerPoint which creates duplicate shapes at the position where each of the selected motion path animation ends. Using this tool you can quickly determine the end position of the animation at design time.