You can use the join node...
Demo
[{"id":"8ccddb9a.a55f38","type":"inject","z":"30e2e9e0.eeaab6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"hi","payload":"100","payloadType":"num","x":1330,"y":880,"wires":[["47b769c5.cb0e28"]]},{"id":"47b769c5.cb0e28","type":"join","z":"30e2e9e0.eeaab6","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":1490,"y":900,"wires":[["0e7a60dec064d06d"]]},{"id":"f9afb265.b11b7","type":"debug","z":"30e2e9e0.eeaab6","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":1770,"y":900,"wires":[]},{"id":"2d269127.4f04ce","type":"inject","z":"30e2e9e0.eeaab6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"lo","payload":"15","payloadType":"num","x":1330,"y":920,"wires":[["47b769c5.cb0e28"]]},{"id":"0e7a60dec064d06d","type":"function","z":"30e2e9e0.eeaab6","name":"","func":"msg.topic = \"diff\"\nmsg.payload = msg.payload.hi - msg.payload.lo;\nreturn msg;","outputs":1,"noerr":1,"initialize":"","finalize":"","libs":[],"x":1620,"y":900,"wires":[["f9afb265.b11b7"]]}]
or you can use a function to grab the globals and do the math
[{"id":"f9afb265.b11b7","type":"debug","z":"30e2e9e0.eeaab6","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":1750,"y":900,"wires":[]},{"id":"0e7a60dec064d06d","type":"function","z":"30e2e9e0.eeaab6","name":"","func":"msg.topic = \"diff\"\n\nvar hi = global.get(\"hi\");\nvar lo = global.get(\"lo\");\n\nmsg.payload = hi - lo;\n\nreturn msg;","outputs":1,"noerr":1,"initialize":"","finalize":"","libs":[],"x":1580,"y":900,"wires":[["f9afb265.b11b7"]]},{"id":"95b54c27bb57d871","type":"inject","z":"30e2e9e0.eeaab6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"hi","payload":"100","payloadType":"num","x":1430,"y":800,"wires":[["6c1a1eeb3b1427fb"]]},{"id":"431f7653eae82780","type":"inject","z":"30e2e9e0.eeaab6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"lo","payload":"15","payloadType":"num","x":1430,"y":840,"wires":[["b8928fa8aec87fab"]]},{"id":"6c1a1eeb3b1427fb","type":"change","z":"30e2e9e0.eeaab6","name":"","rules":[{"t":"set","p":"hi","pt":"global","to":"payload","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":1590,"y":800,"wires":[[]]},{"id":"b8928fa8aec87fab","type":"change","z":"30e2e9e0.eeaab6","name":"","rules":[{"t":"set","p":"lo","pt":"global","to":"payload","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":1590,"y":840,"wires":[[]]},{"id":"fb2b743f2ccbdfe6","type":"inject","z":"30e2e9e0.eeaab6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"true","payloadType":"bool","x":1430,"y":900,"wires":[["0e7a60dec064d06d"]]}]
You could even use a JSONata expression in a change node.