Yes, the reserved Hot-Join Address (7’h02) is safe even if multiple I3C Targets all simultaneously attempt to emit a Hot-Join Request. If multiple I3C Targets do join the I3C Bus and all become eligible to emit the Hot- Join Request (per specification Section 5.1.5) at the same time, then they would be expected (and allowed) to all emit the Hot-Join Request at the same time. Since a Hot-Join Request is a special form of the In-Band Interrupt Request with no data payload (i.e., no Mandatory Data Byte), multiple I3C Targets may all emit this request at the same time, provided that they are all eligible to do so.
Example: As one I3C Target pulls SDA Low to initiate the START Request and drive the 7’h02 Hot-Join Address into the Arbitrable Address Header, and the other eligible I3C Targets see this activity while they are eligible, they would also emit the same Hot-Join Address and behave accordingly. This could be useful if one such I3C Target supported the standard Hot-Join method and waited for the Bus Available condition, while another such I3C Target only supported a passive Hot-Join method but was waiting for another I3C Device to initiate a START Request.
Upon receiving the Hot-Join Request and responding with ACK, the Controller sends the ENTDAA CCC (per specification Section 18.104.22.168) to signal its intent to start the Dynamic Address Assignment procedure and assign Dynamic Addresses to all I3C Targets that have not yet received one. Since the Dynamic Address Assignment procedure inherently supports detection of multiple I3C Targets and uses Arbitration to select one I3C Target at a time (i.e., for each iteration of assignment), the Controller should continue the Dynamic Address Assignment procedure so it can catch all eligible I3C Targets that emitted the Hot-Join Request together (as well as any that might have previously emitted the Hot-Join Request).
See Q17.10, "What has changed regarding Hot-Join in I3C v1.1.1?" for more information about Hot-Join Requests and the specification clarifications that are new for I3C v1.1.1.