So even if you are a warm to average sleeper, who can easily do 0F on the ground with a pad and high quality sleeping bag rated to 0F, it is really hard to say if you will be able to pull it off with a TQ and UQ rated at 0F.
But, not trying to discourage you, because many a person has slept warm at or even below the ratings. In fact, I myself have done it, although I had a real hard time at first making a sleeping bag get anywhere near it's rating when using it as a TQ. Drafts got me every time. But I do much better now, whether with a bag or actual TQ.
So, if you are an average to warm sleeper, I think you can indeed go to zero with a zero rated UQ + TQ. But, you better not have even a small draft, or you can forget it. And Oh yes, don't forget head warmth. A lot of the rating of a sleeping bag is obtained by a draft free closure, with neck collars and a nice thick hood that can close down to a small breathing hole. If you try to do the same temps with a similarly rated TQ but without equivalent head/neck warmth, you won't do so well.