This is how I would solve the problem

Flipping the sodium equation as you did gives:

Na (s) ----- Na+(aq) + e- gives E = 2.71V

Flipping the second equation gives you the following:

Co2+ ------ Co3+ (aq) + e- gives E = -1.82

Look at the format of the equations, isnt there something wrong? How is it possible to have two oxidations occurring??? Oxidation always occurs at the anode, and reduction occurs at the cathode. You would be losing solid sodium at the anode, and obtaining solid Co at the cathode. This would all be from electron transfer. Therefore, you cannot flip the Co equation, and the E cell = 2.71 + 1.82