And how might such a comparison look?
best I can think of would be two separate line graphs. Would it be possible to create timelines/line graphs using filemaker pro 11?
FMP 11 and 12 can generate line graphs using the chart tool.
Whether you can get a chart to look the way you want it will depend on how you go about generating to data series to represent your time lines. And you may need to generate additional "placeholding" records to get a reasonably accurate distance between dates.