Exegesis:

duo lēstas (cf. 11.17) ‘two bandits,’ ‘two robbers’: perhaps, like Barabbas, insurrectionists.

ek dexiōn (cf. 10.37) ‘on the right.’

ek euōnumōn (cf. 10.40) ‘on the left.’

Translation:

With him must be translated spatially, but with caution, e.g. ‘at the same place.’ A literal rendering of ‘with him’ might imply three men on one cross.

One on his right and one on his left is somewhat elliptical, therefore requiring some additions in certain languages, e.g. ‘they crucified one man at Jesus’ right side and another man was crucified at Jesus’ left side.’ In some circumstances, however, a much more natural and meaningful translation would be ‘one on each side of Jesus.’

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .