Appending plots in LTspice
There is not an easy way to do this in LT spice.
A very unorthodox way (that I have used in the past) is export the file and then use a PWL source to read a file (that you have exported). I can't remember if I had to write a script to translate the PWL.
A better way would be to run the Lt spice file, export, and then plot both signals in excel, matlab or python matplotlib.
The best way is to figure out how to use .step to run both of the circuits in the same simulation. With b-sources there are many clever ways to use a voltage source to switch other b-sources to change compoent values with if statements ect.