diff --git a/modules/s2sTesting.js b/modules/s2sTesting.js
index ae68fc3b9f6..eccc3aa1f8a 100644
--- a/modules/s2sTesting.js
+++ b/modules/s2sTesting.js
@@ -11,6 +11,7 @@ s2sTesting.CLIENT = CLIENT;
 
 var testing = false; // whether testing is turned on
 var bidSource = {}; // store bidder sources determined from s2sConfing bidderControl
+s2sTesting.globalRand = Math.random(); // if 10% of bidderA and 10% of bidderB should be server-side, make it the same 10%
 
 // load s2sConfig
 config.getConfig('s2sConfig', config => {
@@ -82,7 +83,7 @@ s2sTesting.getSource = function(sourceWeights = {}, bidSources = [SERVER, CLIENT
   });
   if (!totWeight) return; // bail if no source weights
   // choose a source randomly based on weights
-  var rndWeight = Math.random() * totWeight;
+  var rndWeight = s2sTesting.globalRand * totWeight;
   for (var i = 0; i < bidSources.length; i++) {
     let source = bidSources[i];
     // choose the first source with an incremental weight > random weight
diff --git a/test/spec/modules/s2sTesting_spec.js b/test/spec/modules/s2sTesting_spec.js
index 34de6d9ec38..52377dcabf2 100644
--- a/test/spec/modules/s2sTesting_spec.js
+++ b/test/spec/modules/s2sTesting_spec.js
@@ -1,30 +1,14 @@
 import s2sTesting from 'modules/s2sTesting';
 import { config } from 'src/config';
-import find from 'core-js/library/fn/array/find';
-
-var events = require('src/events');
-var CONSTANTS = require('src/constants.json');
-const BID_ADJUSTMENT = CONSTANTS.EVENTS.BID_ADJUSTMENT;
 
 var expect = require('chai').expect;
 
 describe('s2sTesting', function () {
-  let mathRandomStub;
-  let randomNumber = 0;
-
-  beforeEach(function () {
-    mathRandomStub = sinon.stub(Math, 'random').callsFake(() => { return randomNumber; });
-  });
-
-  afterEach(function () {
-    mathRandomStub.restore();
-  });
-
   describe('s2sTesting.getSource', function () {
     // helper function to set random number and get the source
     function getExpectedSource(randNumber, sourceWeights, sources) {
       // set random number for testing
-      randomNumber = randNumber;
+      s2sTesting.globalRand = randNumber;
       return s2sTesting.getSource(sourceWeights, sources);
     }
 
@@ -93,7 +77,7 @@ describe('s2sTesting', function () {
     describe('setting source through s2sConfig', function () {
       beforeEach(function () {
         // set random number for testing
-        randomNumber = 0.7;
+        s2sTesting.globalRand = 0.7;
       });
 
       it('does not work if testing is "false"', function () {
@@ -155,6 +139,50 @@ describe('s2sTesting', function () {
         expect(serverClientBidders.server).to.eql(['rubicon']);
         expect(serverClientBidders.client).to.have.members(['appnexus']);
       });
+
+      it('sends both bidders to same source when weights are the same', function () {
+        s2sTesting.globalRand = 0.01;
+
+        config.setConfig({s2sConfig: {
+          bidders: ['rubicon', 'appnexus'],
+          testing: true,
+          bidderControl: {
+            rubicon: {bidSource: {server: 1, client: 99}},
+            appnexus: {bidSource: {server: 1, client: 99}}
+          }}});
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          client: ['rubicon', 'appnexus'],
+          server: []
+        });
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          client: ['rubicon', 'appnexus'],
+          server: []
+        });
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          client: ['rubicon', 'appnexus'],
+          server: []
+        });
+
+        config.setConfig({s2sConfig: {
+          bidders: ['rubicon', 'appnexus'],
+          testing: true,
+          bidderControl: {
+            rubicon: {bidSource: {server: 99, client: 1}},
+            appnexus: {bidSource: {server: 99, client: 1}}
+          }}});
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          server: ['rubicon', 'appnexus'],
+          client: []
+        });
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          server: ['rubicon', 'appnexus'],
+          client: []
+        });
+        expect(s2sTesting.getSourceBidderMap()).to.eql({
+          server: ['rubicon', 'appnexus'],
+          client: []
+        });
+      });
     });
 
     describe('setting source through adUnits', function () {
@@ -162,7 +190,7 @@ describe('s2sTesting', function () {
         // reset s2sconfig bid sources
         config.setConfig({s2sConfig: {testing: true}});
         // set random number for testing
-        randomNumber = 0.7;
+        s2sTesting.globalRand = 0.7;
       });
 
       it('sets one bidder source from one adUnit', function () {
@@ -280,7 +308,7 @@ describe('s2sTesting', function () {
         // reset s2sconfig bid sources
         config.setConfig({s2sConfig: {testing: true}});
         // set random number for testing
-        randomNumber = 0.7;
+        s2sTesting.globalRand = 0.7;
       });
 
       it('should get sources from  both', function () {