Move your data classes into external dll, then refer that both places i.e. class Acme sits in your namespace myspace.one and you want it in myspace.two, then move it to myspace.types and refer that both places in myspace.one and myspace.two and they can be aware and handle same types.
Actual data when leaving application boundries for instance should be serialized in one and deserialized in the other for the most easy approach.
There are countless of examples of both techniques, and you will have to do some work yourself :) but here is example of two ways of making data serializable
Binary Serialization with C# and .NET « John Nelson's Blog[
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How to: Serialize and Deserialize JSON Data[
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