Then do both!
#if !NUNIT
using Microsoft.VisualStudio.TestTools.UnitTesting;
using TextFixture = Microsoft.VisualStudio.TestTools.UnitTesting.TestClassAttribute;
using SetUpAttribute = Microsoft.VisualStudio.TestTools.UnitTesting.TestInitializeAttribute;
using TearDownAttribute = Microsoft.VisualStudio.TestTools.UnitTesting.TestCleanupAttribute;
using TestAttribute = Microsoft.VisualStudio.TestTools.UnitTesting.TestMethodAttribute;
#else
using NUnit.Framework;
using TestClass = NUnit.Framework.TestFixtureAttribute;
using TestInitialize = NUnit.Framework.SetUpAttribute;
using TestCleanup = NUnit.Framework.TearDownAttribute;
using TestMethod = NUnit.Framework.TestAttribute;
#endif