Fractal scaling is usually presented as a relationship between aggregate mass and length. Such scaling can also be expressed as a relationship between the lengths of two particles that collide and the length of the resulting aggregate. Emphasizing fractal scaling as a geometric property allows the extension of fractal description to aggregates composed of more than one type of source particle. In particular, it allows the development of more complete models of the role of coagulation in marine ecosystems. The classical aggregation equations can be modified to accommodate a two-dimensional particle size spectrum. This two-dimensional set of equations can be solved using a modification of the sectional approach. Because moving to two-dimensions vastly increases the number of possible interactions and makes solution more computationally costly, simplifications that decrease the allowable interactions considerably speed up the calculations for relatively little loss of accuracy.