How is this single encoder wired?
Your code has two encoders defined but if you have wired pin 5 to 7 and 6 to 8 then the results you get are to be expected.
The problem is the use of the Encoder library, you will have to change the code in that to look at the push button and see what counter to increment.
However the big problem is that reading encoders depends on knowing the last state of the encoder and if you use the same physical encoder then that last state is wrong when you change over so I don’t think your trick will work anyway.