Kyaw Soe Oo's wife Chit Su Win said she planned on attending and was optimistic about the outcome.
"They remain behind bars for one reason: those in power sought to silence the truth", said Adler.
This morning, however, a judge ruled that it was a "reasonable decision" that was "not wrong according to the law" as he dismissed the appeal.
The defence has the option of making a further appeal to the country's supreme court, based in the capital Naypyitaw.
In September last year, a Yangon district court sentenced Wa Lone and Kyaw Soe Oo to seven years in prison for illegal possession of official documents, sparking an worldwide outcry from activists and diplomats in a case seen by many as a test of Myanmar's fledgeling democracy.
Outside the country, the two men have been hailed as heroes and jointly named Time Magazine's Person of the Year 2018, alongside other persecuted and slain journalists.
Before their arrest, the reporters had been working on a Reuters investigation into the killing of 10 Rohingya men and boys by security forces and civilians in Rakhine State during an Army crackdown that began in August 2017.
The appeal was also condemned by Amnesty International, who said the ruling "perpetuates an appalling injustice" while the chief executive of PEN America, Suzanne Nossel, said that "yet again Myanmar's justice system has turned its backs on the principles of rule of law and respect for rights that are the litmus test of democracy".
The trial of the pair was widely criticised, with human rights groups and worldwide governments accusing the Myanmar regime of using the courts to target the two reporters for their reporting on the military-led massacre of Rohingya muslims in the village of Inn Din in Rahkine. "I believe the two will be freed", she said.
Myanmar's military admitted last January that its soldiers took part in the killings, but only after months of denials. The reporters, both Burmese, maintain they were framed.
"The decision to keep these two fearless and innocent Reuters journalists behind bars plays in the hands of the Myanmar authorities' attempt to hide the truth about the atrocities committed in Rakhine", Amnesty's Director of Crisis Response Tirana Hassan said in a statement.
United Nations investigators have called for top Myanmar generals to be investigated for genocide, and criticised the country's de facto leader Nobel Peace Prize laureate Aung San Suu Kyi for failing to stop the attacks.
Jeremy Hunt, the British foreign secretary, who made earlier pledges to champion the journalists' cause, on Friday called on Ms Suu Kyi to take a "personal interest" in the case.