Ahmed and Aden said "all Somali stakeholders" should take part in any conference to forge a national unity government, dismissing the the Somali interim government position that anti-government leaders can only take part as representatives of their clans, not of their movement.
The ICU held south and central Somalia for six months before a combined Ethiopian-Somali force forced them out at the start of the year.
Ahmed and Aden called on Somalis to keep fighting Ethiopian troops and Somalia's interim government.
The planned conference was organised to create a "sense of false legitimacy for the Ethiopian occupying forces", they said.
Meanwhile in Mogadishu, two civilians died and five others were injured as Somali police opened fire on fighters who had attacked their station.
"Two people were killed on the spot when the police opened heavy fire," said Mohamed Omar, a civilian who was passing by when the attack occurred late on Wednesday.
Mohamed Ahmed Abdullahi, another civilian, said: "It was too dark and the police opened fire in self-defence."
He said three people were wounded, with a police officer adding that two others were injured.
Four people, including two police officers, were also killed in the town of Warmahan on Wednesday as police clashed with former militia members who had recently joined the national army.
Such attacks have multiplied since Ethiopia-backed Somali interim government forces cracked down on Muslim fighters last month.