Depth is now.It's difficult to determine exactly what you are trying to understand.
I think that INFXs exist mostly under the surface, where their true selves can't be easily touched by the outside world. However, since INFPs can seem a bit more "bubbly" and "naive" in what they do show the outside world as compared to the INFJs stoic and controlled demeanor, it isn't difficult to get the impression that they are very innocent. Then when they choose to express their true hidden depth, it can be confounding and make them appear inauthentic because it stands in stark contrast to what we had come to percieve as their true selves. However, I wouldn't say it is two-faced or insincere because INFJs also don't show their true colors to the outside world until a degree of trust is established.
As far as the amount of "depth", I wouldn't say that one type could really be described as having more than the other. I would say INFJs express a different type of depth than INFPs, and so it can seem that we "surpass" each other. That would probably be indicative of the Ni,Fe vs. Ne,Fi. You would have to ask BW about that since he is the self proclaimed expert on the "Fi effect". I just don't have the background knowledge in the theory to describe it.