Capacitance of two wires with a grounded wire between
Lets look at a schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

As you can see L2 is now directly grounded instead of beign grounded through C20. If you do your math you can calculate C20 from those 2 measurements. I will leave the rest of it for you to do, have fun.

P.S. This is the representation using concentrated elements, you can create \$\Pi\$ or \$T\$ equivalents and connect them in a cascade to have a better representation of the circuit.