Detecting how many button presses(single tap, double tap etc) with two simultaneous buttons
You SHOULD pay attention to the following issues:

1. Floating input problem:

Symptom: the reading value from the input pin is not matched with the button's pressing state.

Cause: input pin is NOT used pull-up or pull-down resistor.

Solution: Use pull-up or pull-down resistor. See Arduino Button (with pull-up/pull-down)

2. Chattering phenomenon

It should be considered in only some application that needs to detect exactly number of the pressing.

Symptom: Button is pressed one, but Arduino code detects several times.

Cause: Due to mechanical and physical issues, the state of the button (or switch) is quickly toggled between LOW and HIGH several times

Solution: Debounce. See Arduino Button Debounce