Before revision:  We extend the derivation of the 2MA by establishing two advances to previous efforts: a) relative concentrations and b) non-elementary reactions. Both aspects are important in systems biology where one is often forced to aggregate elementary reactions into single stepreactions, and some models use relative concentrations (as a ratio of two concentrations or copy numbers).
After revision:  We extend the derivation of the 2MA by establishing two advances to previous efforts: a) relative concentrations and b) relative concentrations. Often, several elementary reactions are approximated by a single step. Furthermore, practical situations often require the use relative concentrations (as a ratio of two concentrations or copy numbers).