A.) I have a lot to talk about with John about his project.
B.) I have a lot to talk with John about about his project.
Perhaps compare to:
"I want to talk [with John]."
"I want to talk [about Tom's project]."
"I want to talk [with John] [about Tom's project]."
"I want to talk [about Tom's project] [with John]."
If we now look at your original #A version (but with "Tom's project" instead of "his project"), and parse it, we have three PPs:
I have a lot to talk [about] [with John] [about Tom's project].
Which seems, at first blush, to make sense. But notice that the first "about" PP seems to have a gap as its complement, where that gap is understood to mean "a lot (of stuff)": I have a lot to talk about __". And that gap seems to be a relativized element. Compare to:
Those two examples involve infinitival relative clauses (CGEL page 1067). And they are somewhat similar in structure to "I have a lot to talk about __"; for if you replace the "read" in [3.ii] with "talk", and then insert the "about" (similar to the "in" in [3.i]), then you'll have the similar "I've found something interesting (for us) to talk about __".
Now when we try to create your #B version, we can at first add the PP "with John":
- "I have a lot to talk [with John] [about __ ]".
which is fine so far. But then when we add in the last PP:
- "I have a lot to talk [with John] [about __ ] [about Tom's project]".
it seems as if a prosodic pause might be needed between the two "about"s, if this was spoken. And if it was written, then maybe punctuation would be needed. Maybe there is some syntactic reason for this (perhaps because of the presence of the relativized gap?), or maybe it is just me. Someone else might be willing to shed more, or better, grammatical insights onto this topic.