neither of which

[azz]

a. I emailed John and talked to Pete, who agree with me.
b. I emailed John and talked to Pete, who both agree with me.

c. He bought a motorcycle and rented a truck, which aren't really useful for him.
d. He bought a motorcycle and rented a truck, neither of which is really useful for him.

Are the above sentences grammatically correct?

Many thanks

[trolley]

I can't even say why, but in both C & D i would say "useful to" rather than "useful for".
After thinking about it for a while, it seems I say "useful for" when referring to an action that is performed by that useful thing and "useful to" when talking about person or group that benefits from that useful thing.
"A truck would be useful for hauling"
"A tent would be useful for camping"
"Both of those things would be useful to me"
...it would be useful for me to have those things...