THD analysis of an arbitrary expression in LTspice
Mathematically I am not sure whether it is correct what you are trying to do. I suppose you should evaluate the THD of both supplies seperately. However, if you do want to do it in LTSpice, you may use some dummy resistors and an external current source.




In this example, the supply current difference is used as reference to bias another dummy resistor \$R_3\$, yielding:

\$THD_{R1}=43.3\%\$

\$THD_{R2}=47.16\%\$

\$THD_{R3}=0.03\%\$