One limitation to the method mentioned above is if one of the signals doesn't have any values, then no answer is returned. If you still want the value even if one signal is missing than you can try the alternative formula described below. This method works for versions prior to R21.0.40.05.
Here is the formula for 2 signals as shown above:
$signal1.zipWith($signal2, ($s1, $s2) -> max($s1.getValue(), $s2.getValue()))
If you have more than 2 signals, then add additional zipWith() statements:
$signal1.zipWith($signal2, ($s1, $s2) -> max($s1.getValue(), $s2.getValue()))
.zipWith($signal3, ($s1, $s3) -> max($s1.getValue(), $s3.getValue()))
.zipWith($signal4, ($s1, $s4) -> max($s1.getValue(), $s4.getValue()))