Please have a look at figure 15 of:

http://www.analog.com/UploadedF...D694_b.pdf

May be this cleares the problem?

Or do you mean one side of instrumentation amplifier? Well, both inputs show very high common mode input impedances, which can easily be increased further for minimizing error.
By the way, if you are interested in best performance, chaining can not be recommended, at all. It's wise to have as much signal amplitude as possible at shunt, and so shunt should be rather high. But then, there will not be any headroom for chaining.

Kai