Control of two voltage channel by same triggering signal

Hello, I'm trying to control two galvo-mirrors by DAQmx. 

I wanted to use same triggering signal for both of mirrors, so I did the below tasks for both of mirrors. 

Create task->Create voltage channels->Receive clk signal from external source->

Create internal triggering->Write voltage to mirror->start task

The code is as below, but there is an error when DAQmxWriteAnalogF64 function is run sequentially.

Is there anything wrong? Thank you  

int hr;
if((hr = DAQmxCreateTask("",&_hMirrorB)) < 0)
return hr;
if((hr = DAQmxCreateTask("",&_hMirrorC)) < 0)

return hr;
if((hr = DAQmxCreateAOVoltageChan(_hMirrorB, DAQ_AO0,"", _cfg.bscanMin,
_cfg.bscanMax, DAQmx_Val_Volts, "")) < 0)
return hr;

if((hr = DAQmxCreateAOVoltageChan(_hMirrorC, DAQ_AO0,"", _cfg.cscanMin,
_cfg.cscanMax, DAQmx_Val_Volts, "")) < 0)
return hr;
if((hr = DAQmxCfgSampClkTiming(_hMirrorB, DAQ_EXT, EXT_CLK,
DAQmx_Val_Falling, DAQmx_Val_ContSamps, _cfg.N0)) < 0)
return hr;

if((hr = DAQmxCfgSampClkTiming(_hMirrorC, DAQ_EXT, EXT_CLK,
DAQmx_Val_Falling, DAQmx_Val_ContSamps, _cfg.N0)) < 0)
return hr;
if((hr = DAQmxCfgDigEdgeStartTrig(_hMirrorB, DAQ_INT, DAQmx_Val_Rising)) < 0)
return hr;

if((hr = DAQmxCfgDigEdgeStartTrig(_hMirrorC, DAQ_INT, DAQmx_Val_Rising)) < 0)
return hr;
if((hr = DAQmxWriteAnalogF64(_hMirrorB, _cfg.N0, 0, 10.0,
DAQmx_Val_GroupByChannel, (float64*)_bscanWaveform, NULL, NULL)) < 0) {
return hr;
}

if((hr = DAQmxWriteAnalogF64(_hMirrorC, _cfg.N0, 0, 10.0,
DAQmx_Val_GroupByChannel, (float64*)_bscanWaveform, NULL, NULL)) < 0) {
return hr;
}

if((hr = DAQmxStartTask(_hMirrorB)) < 0)
return hr;

if((hr = DAQmxStartTask(_hMirrorC)) < 0)
return hr;