Actually procedure designers simply design to a gradient (3.3% for a SID and 2.5% for a missed approach, unless otherwise specified) - they couldn't care less how many engines you have operating.
That's not the point. The point is that you only have to meet the gradient from, a legal perspective, as long as you are not having an emergency. With an engine failure you are no longer required to meet the gradient (also in the case of the missed approach) - but the operator has to establish a contingency procedure.