You can run multiple instances of the same LabVIEW version by adding the following key in your LabVIEW.ini file
AllowMultipleInstances=True
I have never tried to have more than one L Instance running the application builder at the same time...
Now that I think of if it, I've never even tried to use an instance for coding while another instance is compiling.
LabVIEW crashes so well without trying all that... I rarely run multiple instances at the same time anyway.
The alternative I can suggest is to use a dedicated build machine (virtual or not depending on you IT infrastructure)