How far away can the transformer be from an S-6 Energy Meter?

In my case we're monitoring a duplex-like residence: single building, common-wall, but legally two homes on two different parcels ('zero lot line') - and therefore - two separate electric meters and panels.
So I'm wondering if a single Energy Meter can monitor both units.

The lead length on our 80 Amp CT is about 1 meter.  The signal impedance is low meaning that you can extend that lead length without risk of picking up noise although its still best practice to avoid long parallel runs close to any hot wires.  So for your scenario with a duplex and common wall you might get the reach you need with the standard 1 meter cable but certainly you can extend it to reach the panel on the other side of the wall.  Note that power is computed as the integral of V * I so you need to make sure the breaker you are on in Duplex unit B is on the same leg (phase) as the leg whose voltage you are referencing in unit A.  Finally, the plug used is a 5.5 mm x 2.1 mm DC power jack for which extension leads should be readily available on Amazon as that is a size commonly used in video surveillance systems.