"Mini-Schengen" - agreement with Russia on mutual recognition of visas
The agreement on mutual recognition of visas between Belarus and Russia may come into force until may 20, 2019. This agreement assumes that during the validity of the visa foreigners can enter, leave and stay in the territory of another country, if they have a visa of one of the countries. Therefore, after the entry into force of the agreement, foreign citizens will not need to obtain 2 visas.

But this agreement does not apply to persons who entered Belarus on a visa-free regime, these citizens to visit Russia still need to get a visa. 

Based on: belta.by