(i do not use a customer ID number field because my company only has a few customers that we refer to by name).
I still recommend an ID number field. What will you do if one of your companies changes its name?
You don't have to put either company field inside the portal.
If you have this relationship:
Event Details::CompanyName = Customers::CompanyName
Then when you select or enter a company name in Event Details::Company name, any related recors in Customers will automatically appear in the portal.
If you want to create new customer records in this portal, enable "allow creations of records via this relationship". THen your user can enter a company name and when they start to enter data into the blank bottom row of your portal, a new record will be created and the name in Event Deatails::Comapany Name will automatically be entered into Customers::CompanyName of this new record.