This might happen if the thrust lever resolvers shared a common power supply or the box connected to the thrust lever resolvers had common power or clock circuitry. In the physical realm, I'd look for a broken cable or connector that carried the signals to both engines.
Let's put this one to bed....
#Each dual-redundant channel (A & B) of each engine EEC can supply the
individual left/right thrust lever position resolvers with power (excitation).
Each EEC channel is powered by
individual windings in a permanent magnet dedicated engine-driven alternator (which can function at a mere 5% N3... although some sources quote 8%). There are also
independent sources of power from the main AC busses (Left Bus feeds Left engine EEC, etc) should the dedicated generators fail.
#Separate wiring with large gap between (see a few messages above) the position resolvers.
A picture is worth a thousand words... but all the pics I have are protected by copyright.
Rgds.
NSEU
(Edited for clarity)