A more clear example would be like this :
[{"id":"943b12b4.005428","type":"debug","z":"1b7f56b2.40cf39","name":"Output","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":970,"y":340,"wires":[]},{"id":"21acea57.c848ee","type":"join","z":"1b7f56b2.40cf39","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":true,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"num","reduceFixup":"","x":770,"y":340,"wires":[["943b12b4.005428"]]},{"id":"fa04ca07.305258","type":"inject","z":"1b7f56b2.40cf39","name":"Light Request (Buffer)","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[28,2,172,128,255,100,255,84]","payloadType":"bin","x":280,"y":320,"wires":[["643d6998.2d69b8"]]},{"id":"c4c0f3b0.0f7f5","type":"inject","z":"1b7f56b2.40cf39","name":"Light Reply (JSON)","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"on\":true,\"brightness\":33,\"reachable\":true,\"rgb\":[255,178,76],\"hex\":\"ffb24c\",\"color\":\"orange\",\"colorTemp\":497,\"updated\":\"2020-08-22T20:56:26+02:00\"}","payloadType":"json","x":270,"y":380,"wires":[["ef362bc7.593a28"]]},{"id":"643d6998.2d69b8","type":"change","z":"1b7f56b2.40cf39","name":"","rules":[{"t":"set","p":"topic","pt":"msg","to":"buffer","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":510,"y":320,"wires":[["21acea57.c848ee"]]},{"id":"ef362bc7.593a28","type":"change","z":"1b7f56b2.40cf39","name":"","rules":[{"t":"set","p":"topic","pt":"msg","to":"json","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":510,"y":380,"wires":[["21acea57.c848ee"]]}]
In case our orginal messages dont have a topic. we can use a Change node to set a topic. That topic we use in Change nodes will be used in Join node (with manual settings) to merge the messages.