if two arduinos are connected by I2C and one of them is using an ethernet shield, does that grant both of them access to the internet?
I2C does not automatically share resources between two connected devices. You would not be able to share an Internet connection from a Mega connected via I2C to another Mega which has an Ethernet shield.

However, you could write code for both Megas that implements a two-way communication in which the non-shield Mega sends requests or commands to the one with the Shield, and then passes that data back to the non-shield Mega.

However, you, as the coder, would be responsible for designing such a protocol and implementing it. It would be quite a task probably.