@isTest static void testMethodOne() {
        Test.StartTest();
        Test.setMock(HttpCalloutMock.class, new SFEndpointMock());
        SFEndpointMock.indicatorVar ='success';
        Test.StopTest();
       myClass.performRequest('testData1');
    }

@isTest static void testMethodTwo() {
        Test.StartTest();
        Test.setMock(HttpCalloutMock.class, new SFEndpointMock());
        SFEndpointMock.indicatorVar ='success';
        Test.StopTest();
       myClass.performRequest('testData2');
    }

@isTest
 global class SFEndpointMock implements HttpCalloutMock {
 public static String indicatorVar;
 global HttpResponse respond(HTTPRequest req){
        HttpResponse res = new HttpResponse();
        if(indicatorVar== 'success') {
            res.setStatus('OK');
            res.setStatusCode(200);
            String json = ***JSON BODY HERE***
            res.setBody(json);

      }

else if (indicatorVar== 'success2') {
            res.setStatus('OK');
            res.setStatusCode(400);
            String json = ***DIFFERENT JSON BODY HERE***
            res.setBody(json);

      }
    return res;

if(!Test.isRunningTest()) {

                res = http.send(request);
            }

            else {

                res= MockApiCalls.WebserviceExpectedResponse1();
            }

public class MockApiCalls {
    public static HttpResponse WebserviceExpectedResponse1() {
         HttpResponse res = new HttpResponse();
         String json = ***JSON BODY HERE***
         res.setBody(json);
         res.setStatusCode(200);
         return res;

    }

public static HttpResponse WebserviceExpectedResponse2() {
             HttpResponse res = new HttpResponse();
            String json = ***DIFFERENT JSON BODY HERE***
            res.setBody(json);
             res.setStatusCode(400);
             return res;

        }
  }

@isTest global class SFEndpointMock implements HttpCalloutMock {
    private String indicatorVar;
    global SFEndpointMock(String indicatorVar) {
        this.indicatorVar = indicatorVar;
    }
    global HttpResponse respond(HTTPRequest req){
        ...
    }
}

Test.setMock(HttpCalloutMock.class, new SFEndpointMock('success'));