Do you mean like multiple political parties joining the Communist Party to fight off the Japanese and Nationalist?
Sorry, I'm not that well-verse in the Chinese Civil War.
Yes indeed, the idea of United Front is simple: "联合次要敌人打击主要敌人" or "ally with lesser enemies to take on the main enemy". Meaning there may be political groups you may not agree with for whatever reasons, but you shouldn't take them all on at once. Rather only every engage one faction as the enemy and try to ally with all the rest. In the process of working together you may even be able to turn some of them to your way of thinking.
KMT has actually been on both side of the United Front. During WW2 there was an united front against Japanese where CPC and KMT are on the same side. Even the Eighth Route Army name is actually a National Revolutionary Army unit name and are nominally under overall KMT command.
Once Japan is defeated that incarnation of United Front collapsed with Chongqing Negotiations, CPC then organized a new United Front against KMT and Civil War 2: Electric Boogaloo kicked off.
If we use an analogy from today: under United Front theory the correct thing for US to do would be to ally up with Russia to take on China. This is the same idea you see people like Kissinger and Mearsheimer push and had they had their way it would have been a very difficult and dangerous situation for China. Nevertheless US doesn't heed the wisdom of Mao so they didn't do it to their detriment.