Simply make sure to install them in different directories. The only thing that could "go wrong" would be when two viewers use the same settings file (since they will most probably have different settings names for some specific functions), and when using Linden Lab's stupid uninstaller under Windows (see the Windows installation HOWTO in this forum). The Cool SL Viewer uses its own settings file, so no worry with it (you can even install both the v1.23.5 and v1.19.2 branches of the Cool SL Viewer on the same machine: they each use their own settings file).
Simply make a different launcher/shortcut/menu entry for each viewer...