TeamViewer 6 & 12

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I have 2 PCs (Windows 7-64 bit) side by side. One has TV 12; the other has both TV 6 and 12.. I don't know whether both should be running TV 12, or TV 6, or whether you run 1 of each. At points in the past I had updated TV 6 on both PCs, but now I see both have TV 12., and performance is unpredictable. What should I do to harmonize both PCs?