Combinations of single-top-quark production cross-section measurements and $|f_{\\rm LV}V_{tb}|$ determinations at $\\sqrt{s}=7$ and 8 TeV with the ATLAS and CMS experimentsCombinations of single-top-quark production cross-section measurements and |f$_{LV}$V$_{tb}$| determinations at $ \\sqrt{s} $ = 7 and 8 TeV with the ATLAS and CMS experiments