No matter, if you implemented figure 4-10 or figure 4-11 instrumentation amplifier scheme from the MCP604 data sheet, the input resistance of each input will be almost exactly the ZCM common mode input impedance given in the DC characteristics. As a result of the IAs internal feedback, the impedance of each terminal isn't effectively affected by the other input.
In so far, the input impedance of an instrumentation amplifier can be clearly derived from the datasheet, requiring some general OP circuit knowlegde however. You can refer to a text book, e.g. Gray/Hurst/Lewis/Meyer, that has a detailed discussion of differential and common mode input impedance. But it's possibly too late to start reading it now.