Simultaneous vblank and hblank interruption?

No, the BIOS ISR does not use reti, it uses a simple ei / ret.
Here is the exit of the ISR (INTRET) :

        POP     IX              ;Restore all registers
        POP     IY
        POP     AF
        POP     BC
        POP     DE
        POP     HL
        EX      AF,AF'
        EXX
        POP     AF
        POP     BC
        POP     DE
        POP     HL
        EI
        RET