First of all, a drop-down can only have one selected item.
The way you can do it is with a script, that should be placed as the custom validation script. The general structure of the script should be:
if (event.value=="John Smith") this.getField("Address").value = "John Smith\n1 Main Street\nSpringfield, Arizona"; // or whatever...
else if (event.value=="Jane Doe") this.getField("Address").value = "Jane Doe\n2 Main Street\nAustin, Texas"; // or whatever...
// etc.
else this.getField("Address").value = "";
.
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