Марк

var str1 = 'как я ', str2 = 'яма';

var concat_with_replace = function(str1, str2) {
	var str1_words = str1.split(' '),
		str2_words = str2.split(' '),
		count = 0, last_word = -1, new_word = -1;
	for(var i = 0; i < str2_words.length; i++) {
		for(var j = str1_words.length-1; j >= 0; j--) {
			if(str1_words[j] === str2_words[i] || str1_words[j] == "" || str2_words[i] === "") {
				new_word = j;
				break;
			}
		}
		if(new_word !== last_word + 1 && last_word !== -1 || new_word === -1) {
			break;
		} else {
			new_word = -1;
			last_word = new_word;
			count++;
		}
	}
	var result = str1_words.splice(0 , str1_words.length-count).concat(str2_words);
	for(var i = 0 ; i < result.length; i++) {
		if(result[i] == "") {
			result.splice(i, 1);
			i--;
		}
	}
	return result.join(' ');
}

console.log(concat_with_replace(str1, str2));