25 February 2015 Last updated at 14:27 GMT
They were joined by culture minister Ed Vaizey and shadow international development secretary Mary Creagh.
Nick Robinson says he is not aware of any Labour MPs who are also working as trade union activists - a matter frequently raised by David Cameron - saying this is rather different from MPs who are "sponsored" by unions.
David Cameron and Ed Miliband's clash at Prime Minister's Questions over MPs second jobs is reviewed by Andrew Neil, of the Daily Politics, and BBC Political Editor Nick Robinson.