Based off sawa's comment from this question:
"you cannot have two ...に phrases. Remove either."
And using the same verb "同意する", I looked up the dictionary and found:
1.（人）に同意する - to agree with a person
2.（意見・提案・計画など）に同意する - to agree with an opinion/suggestion/plan/etc.
Let's say I agree with a person about an opinion/suggestion/plan that does not necessarily belong to that person. This way I'm not using the genitive-の to connect the parts together as in
(人)の(意見・提案・計画)に同意する. Instead I'm trying to express that I agree with a person with something.
Deducing from the above dictionary excerpt, I would form a sentence with two
にs (Which I now know is wrong from sawa's comment).
Flawed (pun not intended) attempts at constructing the sentence:
ｘ この計画が彼に同意します (I just realised that this means "This plan agrees with him" (Unless sarcasm is taken into consideration, this sentence cannot make any sense))
(Question) What would be the proper way to construct this sentence?