Sultan al-Anzi, aged 33, and Saud al-Anzi, 35, were arrested and jailed in May 2012, three months after joining the Ahmadiyya group and refusing to abandon their belief, New York-based HRW said.
The group said it urged Saudi King Abdullah back in August 2012 to release the pair, but never received a response.
"Not only have Saudi authorities interfered with the personal beliefs of these two men, but they've left them sitting in jail for two years in legal limbo with no end in sight," said Sarah Leah Whitson, HRW's Middle East director.
"King Abdullah has won acclaim for preaching religious tolerance abroad, but there apparently is no room for tolerance inside his own country."
Under Saudi Arabia's legal system, the penalty for apostasy is death.
HRW said Ahmadiyya activists told it they have had no contact with the two men since their arrest, and do not know their whereabouts or condition.
According to the interior ministry's online prisoner database, both men are in detention but have not been formally charged.
The Ahmadiyya community was founded in British India in the 19th century.
In Sudan, meanwhile, a judge on Thursday sentenced a Christian woman to hang for apostasy.
Born to a Muslim father, Meriam Yahia Ibrahim Ishag was convicted under the Islamic sharia law in force in Sudan that outlaws conversions.
Sudanese Information Minister Ahmed Bilal Osman pointed out his country is not unique in its law against apostasy.
"In Saudi Arabia, in all the Muslim countries, it is not allowed at all for a Muslim to change his religion," he said.