Copy the Y data into a new column, plot both sets of Y data, and apply one fit to each. If you plot the data using a Line plot, you can hide the second Y variable by choosing None from the Show Markers pop-up menu and a Line Style of None (both in Variable Settings). The data will still be there, but only one set of markers will be displayed.
KaleidaGraph is a thoughtfully designed graphing and data analysis application for research scientists, as well as for those in business and engineering fields. It produces publication-quality graphs, and easily converts the most complex data into a functional display.