I cannnot use internet and file access at the same time on ethernet crossover

I have 2 pcs connected via ethernet crossover. main pc is connected to internet via wifi. but i cannot share files and use the internet at the same time on the second pc (it does not support wifi). i want to use the internet and local ethernet at the same time. (im guessing a way to not use dhcp). please tell me what to do

Question by New Warehouse on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.