You can easily use the two versions together, with some caveats. My laptop has LV 6.1, 7.1, 8.0, 8.2.1, and 8.5. The caveats are in your drivers. For example, I cannot use NI-DAQ in 6.1 because the version I have installed for 8.5 doesn't support 6.1. That's not a problem for me. But it could be for you.
The other thing to be very careful with is that every time to open a different version of LV, Windows reassociates LabVIEW file types with the last version you opened. SO if you open 8.5 to test, then close it down and double-click on your 7.1 source, it will open in 8.5. Do a save, and you're in trouble.
What I have ended up doing in my lab is have a multi-boot system with 6.1 and 7.1 on one partition, and another partition with 8.2.1 and 8.5. This way I can get the drivers that I need for all versions. I also have a partition with 7.1 with drivers for one specific customer project, as if I update the drivers on my end, the program will crash on their end. So until I can get them to update their drivers, I need to make sure I never update mine.