If you have a scope that can sample both signals, you may be able to connect the scope to your GPIB card if it supports GPIB and the manufacturer of the scope supplies instrument drivers for LabVIEW.
You can also use a DAQ board. Depending on how close you need to sample the signals with each other, NI has many boards that should fit your need. Probably, sampling both signals at 250 Ksamples/second with a very small amount of time between each signal will give you the info you need.
All of NI's MIO, (multifunction) boards allow you to receive analog signals. There are also boards that allow you to do simultaneous sampling. I would suggest going through the DAQ Advisor to determine what you need. You can find the DAQ Advisor here:
http://www.ni.com/advisor/
Let me know if you have any other questions concerning your application.
Brian
