You aren't supposed to have 2 licences but there is a workaround.
If you are a law abiding citizen then once you have been resident in another country for the x months stipulated in local law you would, naturally, exchange your "foreign" licence for a local one. As part of the process, the issuing authority of the "foreign" licence will be informed of the exchange and cancel the "foreign" licence.
However, as an example, if one should appear to lose one's UK licence before one got around to exchange it for a French licence then one would need to apply for a duplicate UK licence, paying the appropriate fee. One could then exchange the duplicate for a new French licence and the French would send your UK licence back to Swansea to be cancelled. One would have something of a dilemma if one then subsequently found the previously lost UK licence. There would be no point in returning it to Swansea as they will have closed your file. You should really destroy it as it serves no useful purpose, unless one is of a deviant nature and fraudulently flashed it at a gendarme in an attempt to get away with just a fine rather than having points deducted from your French licence. I'm sure nobody has ever done this as it is very naughty.