And if the combined fields are longer than 40 characters?

With Acrobat 4 there was a sample form that one entered the data in fields like tile, first name, mi, last name, abd degrees  and then created a full name from those individual fields and adjusted for missing data.

The doucment level script:

/ Concatenate 3 strings with separators where needed
function fillin(s1, s2, s3, sep) {

/*
Purpose: concatenate up to 3 strings with an optional separator

inputs:
s1: required input string text or empty string
s2: required input string text or empty string
s3: required input string text or empty string
sep: optional separator sting

returns:
sResult concatenated string
*/

// variable to determine how to concatenate the strings
  var test = 0; // all strings null
  var sResult; // re slut string to return
 
// force any number string to a character string for input variables
  s1 = s1.toString();
  s2 = s2.toString();
  s3 = s3.toString();
  if(sep.toString() == undefined) sep = ''; // if sep is undefined force to null

/*
assign a binary value for each string present 
so the computed value of the strings will indicate which strings are present
when converted to a binary value
*/
  if (s1 != "") test += 1; // string 1 present add binary value: 001
  if (s2 != "") test += 2; // string 2 present add binary value: 010
  if (s3 != "") test += 4; // string 3 present add binary value: 100

  /* return appropriate string combination based on
  calculated test value as a binary value
  */
  switch (test.toString(2)) {

  case "0": // no non-empty strings passed - binary 0
     sResult = "";
  break;
 
  case "1": // only string 1 present - binary 1
     sResult = s1;  
  break;
 
  case "10": // only string 2 present - binary 10
     sResult = s2;  
  break;
 
  case "11": // string 1 and 2 present - binary 10 + 1
     sResult = s1 + sep + s2; 
  break;

  case "100": // only string 3 present - binary 100
     sResult = s3;
  break;
 
  case "101": // string 1 and 3 - binary 100 + 001
     sResult = s1 + sep + s3; 
  break;
 
  case "110": // string 2 and 3 - binary 100 + 010
     sResult = s2 + sep + s3; 
  break;
 
  case "111": // all 3 strings  - binary 100 + 010 + 001
     sResult = s1 + sep + s2 + sep + s3; 
  break;
 
  default: // any missed combinations
     sResult = "";
  break;
}

return sResult;
}

And then one could use a custom calculation script for a full address field:

// Full business address including country

function doFullBusinessAddress() {
  var ba = this.getField("business.address.full");
  var bc = this.getField("business.address.citystatezip");
  var bu = this.getField("business.address.country");

  event.value = fillin(ba.value, bc.value, bu.value, ", ");
}

doFullBusinessAddress();