function remain_two_obj(prefix){
 
	this.old = new Array();
	this.prefix = prefix;
 
	this.remain_two = function(cur){
 
		items = document.getElementsByName(this.prefix + '[]');
		for( i = 0, count = 0 ; i < items.length; i++ )
			if( items[i].checked )
				count++;
 
		if( count > 0 && cur.checked == false && this.old[0] == cur.value )
			this.old[0] = this.old[1];
 
		if( cur.checked == false )
			return;
 
		if( count < 2 )
			this.old[count] = cur.value;
 
		else {
			this.old[0] = this.old[1];
			this.old[1] = cur.value;
 
			items = document.getElementsByName(this.prefix + '[]');
			for( j = 0 ; j < items.length ; j++ ){
				if( items[j].value != this.old[0] && items[j].value != this.old[1] )
					items[j].checked = false;
			}
		}
	}
}
 
var cbox = new remain_two_obj('cbox');

<ul>
	<li><label><input type="checkbox" name="cbox[]" value="0" onclick="cbox.remain_two(this)" /> 0</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="1" onclick="cbox.remain_two(this)" /> 1</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="2" onclick="cbox.remain_two(this)" /> 2</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="3" onclick="cbox.remain_two(this)" /> 3</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="4" onclick="cbox.remain_two(this)" /> 4</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="5" onclick="cbox.remain_two(this)" /> 5</label></li>
	<li><label><input type="checkbox" name="cbox[]" value="6" onclick="cbox.remain_two(this)" /> 6<label></li>
	<li><label><input type="checkbox" name="cbox[]" value="7" onclick="cbox.remain_two(this)" /> 7</label></li>
</ul>