With the custom trading styles, you would basically have the option to add more than 1 invoice style, template, and business details - a common use case is the typical “trading as” scenario.
It’s worth noting that the custom trading styles is part of the Power User Subscription, so if you don’t have one of these at the moment, it would be required (if you don’t have one, let me know and I’ll discuss the features in more detail in a private message).
If you have 2 separate businesses, you would typically have 2 QuickFile accounts, as you can’t mix more than one limited company together, or any other business. However, given your situation, it may be best to ask an accountant or business advisor, just to be sure everything is set up right.
There’s an article here which outlines the different features and settings for the custom trading styles, so this may help too.
I hope this helps?