Product addon custom fields
This is very simple. Product addons currently don't have any custom fields like products do.
So if you want to be able to sell two different products at the same time you can only do that by replicating your 2nd product to an product addon. But those addons can't have any custom fields.
For instance if you have an addon called 'Extra bandwith' it would be useful if you could make a dropdown list how much extra bandwith the client can choose from and add another box for comments for example.
I believe there already is a module from Modulesgarden that allows you to link products, maybe that would even be better to then working with product addons.