Josep Rull, once in charge of infrastructure in the Spanish region's separatist executive, and former regional interior minister Joaquim Forn, were all set to refuse food in Catalonia's Lledoners prison, they said in a statement.
“We announce that we are joining the peaceful protest that our colleagues, Jordi Sanchez and Jordi Turull, have already started,” the statement read.
Sanchez, the former head of the influential grassroots ANC independence association, and Turull, a former regional minister, started their hunger strike on Saturday.
They accuse Spain's Constitutional Court of dragging its feet on their appeals against their situation, impeding them from going to the European Court of Human Rights.
The European court rules that a person has to have exhausted all possible appeals at home before taking his or her case to Strasbourg, where it is based.
“Neither our protest nor that of our colleagues is a hunger strike to ask for any treatment of favour or positive discrimination,” Rull and Forn said in their statement.
“It is, simply, the only alternative we have here in jail to make the discrimination and violation of our fundamental rights more visible.”
Spain's Supreme Court in October ordered a total of 18 former Catalan separatist leaders to stand trial over last year's declaration of independence.
Prosecutors are seeking jail terms of up to 25 years on charges of rebellion or misuse of public funds over the failed secession bid.
The sensitive trial is expected to start in early 2019 and will focus on the attempt by Catalan leaders to break away from Spain in October 2017 by staging a referendum despite a court ban and subsequently proclaiming independence.
Nine separatists in all are in pre-trial detention. The seven at Lledoners prison have been held for between eight and 13 months without trial.