Data Path Routing or Switching for AFE circuit
The simplest solution is a high isolation reed relay.  A "Form C" or SPDT relay would allow you to select between two sources.  You could perhaps use a low leakage analog switch like the ADG779, but because analog switches are not passive devices, you could have higher leakage and offset issues.  However, analog switches are generally less expensive.

The relays have > 1000 GOhm isolation.  Remember that at pA leakage levels, you need to worry about issues like surface contamination on the PCB potentially increasing the leakage.