Rafi,
An HP and NI gpib controller should be able to coexist in one system. However, the API's to control the two cards could be very different, depending on how you intend to control them. Are you planning on using VISA, and if so, are you planning to use NI-VISA or Agilent VISA?
Measurement Studio
Measurement Studio augments Microsoft Visual Studio specifically for measurement and automation. Among other things, Measurement Studio gives you environment integration like project wizards and tool windows, user interface controls, and the same NI libraries you find in other languages, such as advanced analysis and DataSocket. For DAQmx, GPIB, and VISA, Measurement Studio allows you to do DAQmx code generation in C++, C# and VB .NET. You can find ou
t more at http://www.ni.com/mstudio.