Exegesis:

proskaleitai (cf. 3.13) ‘he summons,’ ‘he calls to himself.’

hoi dōdeka (cf. 3.14, 16) ‘the Twelve’ as a title, not simply as a number of men (Revised Standard Version ‘the twelve’).

apostellein (cf. 1.2; 3.14) ‘to send off’: a commission and authority to carry out the ministry of Jesus, in accordance with their call as disciples (cf. 3.14-15).

duo duo ‘by the twos,’ ‘two by two’: the use of the cardinal numbers in a distributive sense is not only Semitic but thoroughly Greek as well.

exousian tōn pneumatōn tōn akathartōn ‘authority over the unclean spirits,’ i.e. authority or power to cast them out.

exousia (cf. 1.23; 3.15) ‘authority,’ ‘capacity,’ ‘power.’

ta pneumata ta akatharta (cf. 1.23) ‘the unclean spirits.’

Translation:

For problems in translating the twelve, see 3.14. For authority see 2.10, and for unclean spirits see 1.26.

Called must not be interpreted in the sense of ‘call out to’ or ‘to summon’ (in the legal sense). Moreover, in this context, we are not dealing with the ‘commissioning’ of 3.13. In some languages the equivalent is ‘he spoke especially to the twelve followers’ or ‘he addressed himself to the twelve learners.’

Send them out may be rendered as ‘told them to go out,’ for their going out was in response to a verbal command.

Two by two must be distributive, not collective, i.e. two men went in one direction and two others in another.

Authority over may be rendered simply as ‘power (or ‘strength’) to cast out’ (or ‘to command’). It should be noted, however, that this was a delegated power, not an inherent capacity.

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 .