The vote comes a day after former Conservative foreign secretary Sir Malcolm Rifkindannounced he was ending his career as an MP after being caught by hidden cameras - in a sting that also caught ex-Labour foreign secretary Jack Straw - discussing the possibility of working for a fictitious Chinese company.
Labour tried to embarrass the Tory leader by dragging up an article he wrote for the Belfast Telegraph in 2009, headlined "Double-jobbing MPs won’t get a look-in when I’m in charge", with Miliband telling MPs: "He talked big in opposition about change. He should vote for 1 job not 2."
The Tories derided Miliband's plans as a "gimmick", with a source telling HuffPost UK that it was "as misleading as all his other ones." He added: "It shows exactly why he isn’t up to the job of being Prime Minister."
However, Cameron's attack was focussed on MPs with dual electoral mandates, saying that Northern Irish MPs who don't turn up of "rob voters of a real voice in Parliament".
"Across the UK there are 17 MPs with dual mandates, combining a seat in Westminster with a seat in a devolved institution," he wrote.
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"Of these 17 MPs, 16 are from Northern Ireland: all nine DUP MPs, all five Sinn Fein MPs and two of the SDLP's three MPs."
"Influence in Westminster is based on presence in Westminster. No-one, irrespective of how talented they may be, can for any significant time be a full-time representative in two places."
When journalists highlighted on Twitter that Cameron's attack was not on MPs with second directorships, Labour's press team didn't dispute it, but pointed to other lines were of potential relevance.