You are asking about functions with “more than one var in the signature”. I assume that with “var” you mean “parameter”? Your code on the other hand seems to be calling a function with just one parameter. If lat = 123 and lon = 456, your code would be calling latlon('123,456'). Perhaps you were supposed to write:
webview.runJavaScript("return latlon('" + lat + "','" + lon + "')")