Hello, I'm not so much of a pro user of Axoloti and there might be more straightforward way to achieve it, but here is how I would do it quickly:
I have taken a sine as audio source, but this can be your audio input. I have also used the axoloti button there but you can change these to any gpio pin your buttons are connected to.
Basically, the dry signal, the processed signal n°1 and processed signal n°2 are going to a mux. When no button is pressed, only the dry signal goes to the output. When button1 is pressed, the object last_rising outputs a value of 1 to the mux so it will take i1 and outputs it. When button2 is pressed, the object last_rising outputs a value of 2 to the mux so it will take i2 and outputs it.
I have used klab/logic/last_rising which is a custom object from the community library, there is a version with 8 inputs if you need more.