Ureal Engine中是如何实现一台电脑上插上两个显卡，分别连接两个显示器，并基于D3D11，如何保证两个显示器的视频画面完全同步的，给出简单的代码示例

在UE4中，通过使用DirectX 11的Swap Chain来支持多个显卡和多个显示器。下面是一些简单的代码示例，演示如何在一台电脑上插上两个显卡，分别连接两个显示器，并保证两个显示器的视频画面完全同步。

首先，需要在代码中创建两个Swap Chain，一个对应于每个显示器：

// 定义两个Swap Chain
IDXGISwapChain* SwapChain1;
IDXGISwapChain* SwapChain2;
 
// 获取第一个显卡的Adapter
IDXGIAdapter* Adapter1;
D3D11CreateDevice(NULL, D3D_DRIVER_TYPE_HARDWARE, NULL, 0, NULL, 0,
    D3D11_SDK_VERSION, &Device1, &FeatureLevel1, &Context1);
Adapter1->EnumOutputs(0, &Output1);
 
// 获取第二个显卡的Adapter
IDXGIAdapter* Adapter2;
D3D11CreateDevice(NULL, D3D_DRIVER_TYPE_HARDWARE, NULL, 0, NULL, 0,
    D3D11_SDK_VERSION, &Device2, &FeatureLevel2, &Context2);
Adapter2->EnumOutputs(0, &Output2);
 
// 创建第一个Swap Chain
DXGI_SWAP_CHAIN_DESC SwapChainDesc1;
ZeroMemory(&SwapChainDesc1, sizeof(SwapChainDesc1));
SwapChainDesc1.BufferCount = 1;
SwapChainDesc1.BufferDesc.Width = Width1;
SwapChainDesc1.BufferDesc.Height = Height1;
SwapChainDesc1.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
SwapChainDesc1.BufferDesc.RefreshRate.Numerator = 60;
SwapChainDesc1.BufferDesc.RefreshRate.Denominator = 1;
SwapChainDesc1.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
SwapChainDesc1.OutputWindow = hWnd1;
SwapChainDesc1.SampleDesc.Count = 1;
SwapChainDesc1.SampleDesc.Quality = 0;
SwapChainDesc1.Windowed = TRUE;
IDXGISwapChain* SwapChain1;
Factory1->CreateSwapChain(Device1, &SwapChainDesc1, &SwapChain1);
 
// 创建第二个Swap Chain
DXGI_SWAP_CHAIN_DESC SwapChainDesc2;
ZeroMemory(&SwapChainDesc2, sizeof(SwapChainDesc2));
SwapChainDesc2.BufferCount = 1;
SwapChainDesc2.BufferDesc.Width = Width2;
SwapChainDesc2.BufferDesc.Height = Height2;
SwapChainDesc2.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
SwapChainDesc2.BufferDesc.RefreshRate.Numerator = 60;
SwapChainDesc2.BufferDesc.RefreshRate.Denominator = 1;
SwapChainDesc2.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
SwapChainDesc2.OutputWindow = hWnd2;
SwapChainDesc2.SampleDesc.Count = 1;
SwapChainDesc2.SampleDesc.Quality = 0;
SwapChainDesc2.Windowed = TRUE;
IDXGISwapChain* SwapChain2;
Factory2->CreateSwapChain(Device2, &SwapChainDesc2, &SwapChain2);

接下来，需要在每个帧中，将场景渲染到两个Swap Chain中，然后在两个Swap Chain之间进行同步：

// 渲染第一个Swap Chain
ID3D11RenderTargetView* RTV1;
SwapChain1->GetBuffer(0, IID_PPV_ARGS(&BackBuffer1));
Device1->CreateRenderTargetView(BackBuffer1, NULL, &RTV1);
Context1->OMSetRenderTargets(1, &RTV1, NULL);
RenderScene();
SwapChain1->Present(1, 0);
 
// 渲染第二个Swap Chain
ID3D11RenderTargetView* RTV2;
SwapChain2->GetBuffer(0, IID_PPV_ARGS(&BackBuffer2));
Device2->CreateRenderTargetView(BackBuffer2, NULL, &RTV2);
Context2->OMSetRenderTargets(1, &RTV2, NULL);
RenderScene();
SwapChain2->Present(1, 0);
 
// 同步两个Swap Chain
DXGI_FRAME_STATISTICS Stats1, Stats2;
SwapChain1->GetFrameStatistics(&Stats1);
SwapChain2->GetFrameStatistics(&Stats2);
while (Stats1.SyncRefreshCount != Stats2.SyncRefreshCount) {
    SwapChain1->GetFrameStatistics(&Stats1);
    SwapChain2->GetFrameStatistics(&Stats2);
}

在上面的代码中，RenderScene()函数是渲染场景的函数。同步两个Swap Chain的方法是通过比较两个Swap Chain的SyncRefreshCount属性来实现的。

总之，以上代码示例是一个简单的演示，如果要实现更复杂的多显卡和多显示器的场景，可能需要更多的代码和资源管理技能。