It very depends where and when you are have to do it.
So the simplest way (when target legends are on the page and filled by their values) is to use next code:
pre
//Get values of existing labels.
var labelFirstNameText = Appery("label_firstName").text();
var labelSecondNameText = Appery("label_secondName").text();
//Combine two values together.
var labelFullNameText = labelFirstNameText + " " + labelSecondNameText;
//Populate full name label by resulting value.
Appery("label_fullName").text(labelFullNameText);
/pre
Regards.