Is it possible to calculate the Time of Flight between two Bluetooth devices?
Very unlikely

Bluetooth uses radio, and radio travels at the speed of light.  A 1cm round trip will take less than 100ps. Timing something that short will be tricky, probably you'll want a 10GHz clock, though there are other options.  But even then, Bluetooth isn't designed to instantly echo the radio message.  If you receive, process and re-transmit the message, then the processing delay will be much longer than the time of flight, and will vary randomly by at least the period of the clock used with the Bluetooth chip.

With some clever hardware, it probably is possible to make 100ps accurate time of flight measurements.  But it's hard and trying to combine with Bluetooth is just going to make it harder.  

Real life example: Apple uses the 802.11v 'time of flight' for detecting if an Watch is in proximity to MacBook. The 802.11v paper claims "a mean absolute error of less than 1.33 meters in an ideal environment". For Apple it means they just check if two devices are no farther than 3m from each other.  

As an alternative, consider:


Ultrasound (if you can aim the sensor at the other tag)
Differential GPS (if you don't know orientation)