very easy : use "\n"
String aString1 = "abcd";
String aString2 = "1234";
mSomeTextView.setText(aString1 + "\n" + aString2);
\n corresponds to ASCII char 0xA, which is 'LF' or line feed
\r corresponds to ASCII char 0xD, which is 'CR' or carriage return
this dates back from the very first typewriters, where you could choose to do only a line feed (and type just a line lower), or a line feed + carriage return (which also moves to the beginning of a line)
on Android / java the \n corresponds to a carriage return + line feed, as you would otherwise just 'overwrite' the same line