If two atoms are bonded together, one with six valence electrons, the other with four, how many electrons must they share for both to achieve a full octet?

8 electrons

Explanation:

For the two atoms to have an octet configuration, they must share

8 electrons. Two of the atom with six valence electron will bond with one of the atom with four valence electrons. This is clearly seen when carbon react will sulphur to form carbon disulfide. They form covalent double bond (S=C=S)