Evaluation Board Connection—AD9681-125EBZ and HSC-ADC-EVALDZ，
Yes, the AD9681 can do 8 channel simultaneous sampling. In its power-up-default condition the AD9681 will sample all 8 analog inputs simultaneously.
As you have noted, the AD9681 VisualAnalog canvases only have two channels by default. I have attached an example VisualAnalog Samples canvas that will capture and display time domain waveforms for all 8 channels.
You might need to point this VisualAnalog canvas to the correct FPGA file. Instructions on how to do this are in the AD9681 Evaluation User Guide.
EVALUATING THE AD9681 ANALOG-TO-DIGITAL CONVERTER [Analog Devices Wiki]