It would help to distinguish between (1) antenna detuning because of bridging between the antennas and (2) attenuation because of having a lump of human body next to the antenna. I don't know that that would be directly useful in itself, but it would be interesting. Obvious further experiments: what happens when you add some impedance (resistive and/or capacitive) to the connection between the antennas? what happens if you put a conductive thing, broadly similar to a human hand but better controlled, in various places near the antenna? Etc.

The results of doing a bunch of measurements like this could make it much easier to answer questions like: What is it about your hand or your grip or whatever that determines how much you impair reception on your iPhone? How much of the problem would go away if you put various things (kapton tape, rubber bumper, etc.) between the antenna and your hand?

Just playing around with actual human hands can give some information of that sort, and if you're only going to do a small amount of investigation you probably get more information that way. But for a deeper understanding of what's going on, you'd want to be more scientific and try harder to decouple different failure mechanisms.

(Of course talk is cheap. I haven't done any of that. Not least because I happen not to have an iPhone.)