One - Junaid Hussain of Birmingham - left the UK while on bail pending investigation for violent disorder.
The other, Abu Aziz from Luton, has been convicted in his absence of an attack on a member of the public.
Aziz is an associate of Anjem Choudary, the high-profile former leader of a group banned under terrorism laws.
It is understood that Aziz, 32, fled the UK in March along with another British man, Mohammod Alom. The pair headed to Amsterdam before, according to multiple sources, going on to Turkey - the route for British fighters seeking to enter Syria to join jihadist groups.
While it is not known whether the pair crossed the Turkish-Syrian border, a former associate of Aziz, who wishes to remain anonymous, told the BBC that he believed the Luton man had made it to Syria. He said that he believed Aziz would not be returning, nor pose a threat to the UK.
A month before he disappeared, Aziz posted a lecture on a social media site calling on Muslims to go to Syria.
He said: "Why is it that we have become blinded by the dunya [material world], that we don't see what is happening to our brothers and sisters in Bilaad al Shaam [Syria]? Just because they are far away from us it does not mean they are not our family."
Aziz has not responded to online questions about his whereabouts but in the last few days he has posted messages about the conflict, including a declaration of support for militant group ISIS. No one was available for comment at his home.
Junaid Hussain was arrested in July last on suspicion of violent disorder.
The BBC has learned that he left the UK while he was still under criminal investigation for the alleged violence. West Midlands Police later decided he would face no further action.
In a statement, the force said that Hussain had not been required to attend a police station or surrender his passport while inquiries were continuing.
Aziz regularly attended protests by Muslims Against Crusades, a group headed by Anjem Choudary before its 2011 ban.
In a report published last year, campaign group Hope Not Hate estimated that dozens of the British fighters in Syria had a connection with Anjem Choudary's group.
The report said the number of fighters from Aziz's hometown of Luton may be in double figures, although that has not been independently verified.
Mr Choudary told the BBC "I know Abu Aziz very well. He was very active when he was here... He was always concerned about Muslims abroad."
He denied knowing where Aziz was or playing any role in sending Britons to fight in Syria, adding: "No one has asked me before they go abroad. They don't need my permission."
Earlier on Wednesday, the prime minister warned that the threat from jihadist fighters in Iraq and Syria would "come back to hit the UK" if it were not tackled.
That warning came after Foreign Secretary William Hague said that as many as 400 British citizens may be fighting in Syria, including some with ISIS, the insurgent force now fighting attacking Iraq.
Abu Aziz was tried in his absence at the Old Bailey after being charged with affray after an incident following a demonstration outside the US Embassy in May last year.
Prosecutors say he left the demonstration to man a prayer stall - but then punched a passer-by who was walking away from a pub towards Oxford Street.
Aziz and other men set upon Andrew White, bundling him to the ground and then punching and kicking him. He was left bleeding from the head and mouth.
Aziz and his nine co-defendants who pleaded guilty to affray will be sentenced on Thursday and Friday.
Several of them were previously convicted for their role in an angry demonstration in 2009 by Muslims Against Crusades in Luton against British soldiers returning from Afghanistan.
Two of Aziz's co-defendants, Qadeer Ahmed and Yousaf Bashir, broke their bail conditions to fly to the Middle East, although it is not known why. They returned to the UK before the trial began.
Raffaello Pantucci of the Royal United Services Institute said that many British fighters in Syria were trying to remain anonymous to avoid attention from the security services.
"It is surprising, however, that someone from [the Muslims Against Crusades network] would stay so quiet given their tendency to like to take a very aggressive and confrontational style," he said.
"However, they are also aware of the legislation in the United Kingdom and would have noticed how many of those who are coming back to the UK are being arrested."
Two men who were under criminal investigation have absconded from the UK intending to join jihadists in Syria, the BBC has learned.