Kevin Lau, a former editor of the liberal Ming Pao newspaper, was hacked with a cleaver in broad daylight last month by two men who fled the scene on a motorbike.
"Police further arrested a 55-year-old man and a 37-year old woman in the small hours of today," city police said in a statement.
The force had yesterday announced the arrests of nine men, including two suspects from the Chinese mainland with links to triad organised crime gangs.
Seven other "accomplices" were arrested across Hong Kong, he said.
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Lau remains in hospital following the assault on February 26, in which he was struck six times on the back and legs with a cleaver, leaving wounds including a 16-centimetre-long (six-inch) gash.
The attack came just weeks after Lau was sacked from the top job at Ming Pao and replaced with an editor widely seen as pro-Beijing.
His overthrow triggered protests over media freedom in Hong Kong, with concerns mounting that Beijing is seeking to tighten control over the semi-autonomous region.
But Tsang said yesterday that police had yet to find a motive or establish a connection between the attack and Lau's journalism.