I was able to get two of these sensors to work like this. Some control of the state of the GPIO1 pin can be achieved by using the SYSTEM__MODE_GPIO1 register (documented on page 55 of the VL6180X datasheet). By setting system__gpio1_select to GPIO Interrupt output, the state of the output pin can be set using system__gpio1_polarity.
Our breakout board has pullup resistors on both the GPIO0/CE and GPIO1 pins, so when connecting GPIO1 of one board to GPIO0/CE of the next, GPIO1 will be high when system__gpio1_select is set to OFF (high impedance). The sensor is meant to toggle the state of the GPIO1 pin when taking measurements (subject to some interrupt conditions), though it seems like setting system__gpio1_select to OFF disables this. It is unclear to me whether the status of other registers in the sensor like the SYSTEM__INTERRUPT_CONFIG_GPIO register might affect this, so you should be careful.
The following code shows what I did for two sensors, and the basic template should be expandable for more:
sensor.writeReg(0x011, 0b00110000); // GPIO1 LOW (ActiveHigh InterruptOutput)
sensor.writeReg(0x011, 0b00000000); // GPIO1 High Impedance and pull-ups keep pin HIGH
sensor2.writeReg(0x011, 0b00110000); // GPIO1 LOW (ActiveHigh InterruptOutput)
sensor2.writeReg(0x011, 0b00000000); // GPIO1 High Impedance and pull-ups keep pin HIGH