Income and Expenditure Categories

[gkpattison]

I am a new user just setting up my accounts for a youth organisation.
I read in the documentation that you should not use the same category for income and expenditure as it will affect balance calculations. Is it allowed to use a single category but separate sub-categories for income and expenditure?
For example:
Category>Event One
Sub Category>Event Income
Sub Category>Event Expenditure
Category>Event Two
Sub Category>Event Income
Sub Category>Event Expenditure
or would I need to set up four categories for the above?