Hello,
As you already may have understood, ST25DV cannot be accessed at the same time from RF and from I2C.
For FTM, this has to be taken into consideration.
On I2C side, I recommend using the interrupt to synchronize on the RF. The interrupts RF_PUT_MSG and RF_GET_MSG are triggered when the RF has written or read a message. At this moment, the RF command has ended and the I2C can read the status registers and read or write the message in the mailbox.
On RF side, this is trickier as there is no interrupt possible. The RF reader must poll the status register MB_CTRL_Dyn to know if the I2C has read or write the message. This has to be done at with some temporization so that the I2C has finished reading or writing the message and is no more active so that there is no collision.
It can go from a few ms (5ms for example) up to several ms (may be 20 or 40ms) depending on the speed on the I2C side. This is very dependent on the implementation on the MCU side (MCU speed, FW, I2C bus speed etc.) so there is no generic value I can recommend. You have to experiment to find the value that is good for you.
Best regards.