One suspect, aged 32, is accused of recruiting for the Islamic State group, while the other, aged 39, faces allegations of being a recruiter for "terrorist organisations in Syria".
One of the two is from the Osh region of Kyrgyzstan, an area known for the number of recruits it has provided for Islamic State in Syria and Iraq.
Fourteen people died and dozens were injured in the April 3 blast which has been blamed on 22-year-old Akbarjon Djalilov, thought to be a Russian national born in Kyrgyzstan, who died in the blast.
Eight people -- all from mainly Muslim Central Asia -- have been detained in Moscow and Saint Petersburg over alleged involvement in the attack.
As of the beginning of April, police in St Petersburg had arrested seven nationals of Central Asian countries on suspicion of being "terrorist" recruiters.
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