AVR ATmega: can I trigger external interrupt by setting corresponding INTF flag bit?
Nope.

Writing a 1 to the flag will clear it. Writing a 0 will do nothing.



However, if you are using avr-libc and want to have two (or more) vectors having exactly the same code, this is possible. You use interrupt aliasing. An example (from here):

ISR(PCINT0_vect){  
    ...  
    // Code to handle the event.
}

ISR(PCINT1_vect, ISR_ALIASOF(PCINT0_vect));


In this example both PCINT0 and PCINT1 will share the same interrupt code - both entries in the vector table will point at the same function. This may be sufficient for your needs.