How to solve error -61202 with using two FPGA references

I met a problem when I programmed for myRIO in LabVIEW. I want to use myRIO to acquire audio data and produce PWM signals in the same time, to form a closed loop control for a robot arm with servo motor. But both the data acquisition and PWM signal production need to use FPGA reference, an error will happen, the error code is -61202. How can I achieve the function without making the error.