It is perfectly normal to do what you expect. Just make sure you are not referencing newer versions of the DLL in the old version. For example, you are not allowed to refer to a class library project with a base of 4.0 to 3.5 framework projects.
This is the only thing you need to keep in mind in general.
There may also be other considerations, but it depends on the type of your project and how you want to use the project with each other.
So, if this answer does not contain details about what you want to know, edit your question with more detailed information on the types of projects.
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