Field Mapping possibly?

Hi 

The basic concept of what i am trying to do is for custom entities to show the same data on different forms. 

Example:

on my lead form i have two custom field, Related sales orders and related invoices. users manually add these from existing or when they create new ones.

i have created the same fields with the same configurations on my opportunity form, how do i get it to display the same information on the different entities.