If I am using both statements at the same time
As Jon Skeet mentions in his comment, you should only be using one test framework. They will not play nicely together. If you want to use NUnit, you should,
Microsoft.VisualStudio.QualityTools.UnitTestFrameworkfrom your project references.
Your unit tests will only end up running under one of the test adapters, MSTest or NUnit. The asserts for each test framework throw different exceptions on assert failures and each test runner expects its own exceptions to be thrown. If you try to use an MSTest assert in your tests and your test fails, NUnit will not recognize the exception thrown as an assertion exception, so it will report it as an error as opposed to a test failure.
You likely ended up with both test frameworks because you created a Unit Test Project in Visual Studio, then added NUnit to it. NUnit test projects should just be a regular Class Library. Another option is to install the NUnit Templates Extension for Visual Studio which will add an NUnit 3 Unit Test Project template to Visual Studio.