There shouldn't be a problem with LV itself (although officially they won't be supported on the same version of Windows), but if you have NI hardware drivers, like DAQmx, you will have a problem because you can only install one version and there is no version which supports both 6.1 and 2012.
If you want to be safe, I would suggest installing 6.1 on a virtual machine (Windows 7 pro should have that built-in and you can also find other solutions, like Virtualbox).
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