Well the 12 lead is more finnessed than that - essentially without a 12 lead your ability to definitively rule out an acute heart attack is completely impaired. 5 lead ECGs will still miss (some high percentage, above 10% and below 50%) of heart attacks.

A one (two?) lead ecg as you describe is not going to tell anyone what is going on with ST segments, particularly as you would be only looking at your aVR or aVL(depending on which way you sum the currents).