Anthony and Theodosius were sainted founders of the Russian and Ukrainian monasticism. They established the Kiev Pechersk Monastery in the mid-11th century. See Saint Anthony of Kiev

Saint Anthony of Kiev

Anthony of Kiev was a monk and the founder of the monastic tradition in the Kievan Rus'. Also called Anthony of the Caves he, together with Theodosius of Kiev, co-founded Kiev Pechersk Lavra ....

 and Theodosius of Kiev

Theodosius of Kiev

Theodosius of Kiev is an 11th century saint who brought Cenobitic Monasticism to Kievan Rus' and, together with St Anthony of Kiev, founded the Kiev Caves Lavra...

 for details.

In the Orthodox Church, their names are often listed together and they are commemorated together on September 2, while each has his own feast day as well (July 10 for St. Anthony and May 3 for St. Theodosius).