2nd week on 150mg bupropion taken twice daily. Having heart palpitations. What shall I do?

Decrease dosage. Consult the doctor who prescribed to you and explain possible side effect and alternative care.
Dose change? Perhaps you should take the bupropion once a day for a little while then try the twice daily dosing. If you consume caffeinated beverages, consider reducing or eliminating them. Talk with your doctor about the palpitations so he can address the issue. It also depends on how often you get the palpitations, how severe they are, and how long they last.
Discuss. Discuss this with the prescribing physician. He/she may try you on a lower dose. If the palpitations are not due to bupropion, avoid caffeine and alcohol to see if that helps.