It is the second time in little over a month the group has protested at Ms Harman's home in south London.
Scotland Yard said police were called at 6.22am. It is not known if Ms Harman was at home when the incident began.
It is believed the pair are dressed as superheroes, with one in a Batman costume and the other dressed as Spiderman.
Last month two protesters, Jolly Stanesby and Mark Harris, both from Devon, scaled the roof of Ms Harman's home dressed as superheroes.
Mr Stanesby spent more than 24 hours on the Cabinet minister's roof in a protest that began on June 8. The pair were bailed by police until July 16 pending further inquiries.
The group said at the time it wanted to highlight the fact that fathers were being made redundant emotionally in the courts, and now biologically in the new Human Fertilisation and Embryology Bill.
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