External interrupt on atmega328p
The obvious: Connect two buttons to one interrupt and use a different pin and code to figure out which one was actually pressed in the Interrupt Handler.

That's pretty standard procedure in hardware design.

Also, note that humans are incredibly slow compared to microcontrollers. It's usually totally unnecessary to use an interrupt line for something like a button unless it's for waking up from extreme deep sleep modes (which the ATMega 328 doesn't have, as far as I know).

So, unless you really need to have high-speed interrupts, it's usually not desirable to use interrupt pins for button IO – buttons ring, and that means that pressing the button once will trigger a whole torrent of interrupts, of which you only want the first one.

Instead, simply loop your code, and occasionally check your IO pins.