Great course. I have multiple clients trying to communication via MQTT. Either client working alone works perfectly. When the second unit brought on-line the first unit quits communicating.
Which course are you following? Basically, each MQTT client (ESP32, ESP8266, or Arduino) needs to have a unique client name.
For example, if you open this MQTT client example: https://github.com/RuiSantosdotme/Home-Automation-Course/blob/master/code/v2/mqtt_esp8266_final.ino
Lines: 180 and 249 should have the same unique name for each MQTT client.
Change to something like this in Lines: 180 and 249:
I hope that helps.