The reaction has to be carried out in 2 steps:

Step 1 : Conversion of Methyl bromide to ethane

This conversion can be facilitated by wither Na/Dry ether (wurtz reaction) or Zn metal i.e. Frankland reaction

Step 2: Bromination of ethane

Ethane when subject to Bromine in presence of light undergoes bromination leading to formation of ethyl bromide.
2CH3Br −→−−−−Na/ Ether CH3−CH3 −→−−−Br2 / hνCH3CH2Br