If you want to get a little fancier, here's the current iteration of the scripts we have been using to which add the Task/RITM number for reference, prevent recursion, and won't copy duplicates when users repeatedly enter the same comment for whatever reason.

The first rule, on the sc_req_item table after Update when Additional comments changes:

(function executeRule(current, previous /*null when async*/) {
	//To Copy the Additional Comments from RITM to SCTASKs
    var compare = '';
	var ritm_comment2 = '';
	var ritm_comment = '';
	var task_comment2 = '';
	var task_comment = '';
	
	ritm_comment = current.comments.getJournalEntry(1);
	
	//Remove timestamp and name from additional comment
	var regex= new RegExp('\n');
	var i = ritm_comment.search(regex);
	if (i>0) {
		ritm_comment2 = ritm_comment.substring(i+1, ritm_comment.length);	
	}
	
	var task_gr = new GlideRecord('sc_task');
	task_gr.addQuery('request_item', current.sys_id);
	task_gr.addQuery('active', 'true');
	task_gr.query();
	while (task_gr.next()) {
		task_comment = task_gr.comments.getJournalEntry(1);
		
		//Remove timestamp and name from additional comment
		var i1 = task_comment.search(regex);
		if(i1 > 0) {
			task_comment2 = task_comment.substring(i1+1, task_comment.length);
			var numberstring = current.number + ' - ';
			var regex2= new RegExp(numberstring);
			var i2 = task_comment2.search(regex2);
			if(i2>=0) {
				task_comment2 = task_comment2.substring(i2+numberstring.length, task_comment2.length);
			}
		} else {
			task_comment2 = 'empty';
		}
		var numberstring2 = task_gr.number + ' - ';
		var regex3= new RegExp(numberstring2);
		var i3 = ritm_comment2.search(regex3);
		if(i3>=0) {
			ritm_comment2 = ritm_comment2.substring(i3+numberstring2.length, ritm_comment2.length);
		}
		
		compare = ritm_comment2.indexOf(task_comment2);
				
		if (compare != 0) {// if no match found
			task_gr.comments = current.number + ' - ' + ritm_comment2.trim();
			task_gr.update();
		}
	}	
})(current, previous);

The second rule, on the sc_task table after Update when Additional comments changes:

(function executeRule(current, previous /*null when async*/) {
	//to copy the Additional Comments from SCTASK to RITM
	var compare = '';
	var task_comment2 = '';
	var task_comment = '';
	var ritm_comment2 = '';
	var ritm_comment = '';
	
	task_comment = current.comments.getJournalEntry(1);
		
	//Remove timestamp and name from additional comment
	var regex= new RegExp('\n');
	var i = task_comment.search(regex);
	if (i>0) {
		task_comment2 = task_comment.substring(i+1, task_comment.length);
	}
	
	var ritm_gr = new GlideRecord('sc_req_item');
	ritm_gr.addQuery('sys_id', current.request_item);
	ritm_gr.query();
	if(ritm_gr.next()) {		
		ritm_comment =ritm_gr.comments.getJournalEntry(1);
			
		//Remove timestamp and name from additional comment
		var i1 = ritm_comment.search(regex);
		if(i1 > 0) {
			ritm_comment2 = ritm_comment.substring(i1+1, ritm_comment.length);
			var numberstring = current.number + ' - ';
			var regex2= new RegExp(numberstring);
			var i2 = ritm_comment2.search(regex2);
			if(i2>=0) {
				ritm_comment2 = ritm_comment2.substring(i2+numberstring.length, ritm_comment2.length);
			}
		} else {
			ritm_comment2 = 'empty';
		}
		
		var numberstring2 = ritm_gr.number + ' - ';
		var regex3= new RegExp(numberstring2);
		var i3 = task_comment2.search(regex3);
		if(i3>=0) {
			task_comment2 = task_comment2.substring(i3+numberstring2.length, task_comment2.length);
		}
		
		compare = task_comment2.indexOf(ritm_comment2);
		
		if(compare != 0) {  // If no exact entire match found
			ritm_gr.comments = current.number + ' - ' + task_comment2.trim();
			ritm_gr.update();
		}		
	}
})(current, previous);