some questions about metal synchronizing the resources using MTLFence

Hi guys,I use MTLFence for synchronizing the resources,but I didn't get what I wanted. I want the oneTriangleTex having one triangle and the twoTriangleTex having two triangles,but they have the same two triangle, I don't know if the usage is correct. If not, what should I do? code snippets:

texDescriptor.hazardTrackingMode=MTLHazardTrackingModeUntracked;
texDescriptor.storageMode=MTLStorageModePrivate;
id<MTLTexture> oneTriangleTex = [_device newTextureWithDescriptor:texDescriptor];
id<MTLTexture> twoTriangleTex = [_device newTextureWithDescriptor:texDescriptor];

id<MTLFence> endPrimFence = [_device newFence];
id<MTLFence> endCopyFboFence = [_device newFence];

id <MTLCommandBuffer> cmdBuffer = [_commandQueue commandBuffer];
 
//render,using MTLLoadActionClear for color attachment
id <MTLRenderCommandEncoder> renderEncoder = [cmdBuffer renderCommandEncoderWithDescriptor:_descriptor];

//draw triangle a
[renderEncoder drawPrimitives:...];
[renderEncoder updateFence:endPrimFence afterStages:MTLRenderStageFragment];

[renderEncoder waitForFence:endCopyFboFence beforeStages:MTLRenderStageFragment];

//draw triangle b
[renderEncoder drawPrimitives:...];
[renderEncoder updateFence:endPrimFence afterStages:MTLRenderStageFragment];

[renderEncoder endEncoding];
 
// blit
id<MTLBlitCommandEncoder> biltEncoder = [cmdBuffer blitCommandEncoder];

[biltEncoder waitForFence:endPrimFence];
//copy fbo color to oneTriangleStoreTex
[biltEncoder copyFromTexture:attachmentTex
                    sourceSlice:0
                    sourceLevel:0
                    sourceOrigin:srcOrig
                    sourceSize:cpSize
                    toTexture:oneTriangleTex
                destinationSlice:0
                destinationLevel:0
                destinationOrigin:dstOrig];
[biltEncoder updateFence:endCopyFboFence];

[biltEncoder waitForFence:endPrimFence];
//copy fbo color to twoTriangleStoreTex
[biltEncoder copyFromTexture:attachmentTex
                    sourceSlice:0
                    sourceLevel:0
                    sourceOrigin:srcOrig
                    sourceSize:cpSize
                    toTexture:twoTriangleTex
                destinationSlice:0
                destinationLevel:0
                destinationOrigin:dstOrig];

[biltEncoder endEncoding];
 
[cmdBuffer commit];