The idea behind multiple designer support is to enable collaborative process modeling. Each workflow designer works differently, although they store the process definition as .kprx. If you take Visio for example, it embeds the process definition inside the .vdx file which is the Visio file format. VS.NET has a solution, project concept and stores .kprx files directly under the project system. Import and Export functions are provided in the Visio plug-in to sync between the VS.NET and Visio designers. There is some manual effort required.

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To expand upon Jey's explanation.  It can certainly be done, but keep in mind the collaborative process modeling is meant to be structured as "hand-offs" back and forth between the people using Visio and VS.  If they are working on the same process at the same time each within their own respective tooling (Visio/VS), there is no provisions made to merge together the concurrent changes.

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