When you create test plans for MTM, you can specify configurations for it and which one will be the default. Therefore, when you add new requirements, they automatically accept the default configuration. However, you can always change it by assigning another available configuration for any necessary requirements. My point is that the test script, owned by two different User Stories, when assigned to the test plan, contains additional information, which is the configuration that it will use to validate it.
So, if your test case A is assigned to user stories A and B, and these requirements were assigned to the same test plan , but they have different configurations, it is very possible that one test case is an instance for failure and the other is.
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