If you want to claim work from home expenses for both your employment income and your freelance (self-employment) income, you would need to use both. You just have to make sure that you only claim the portion of your expenses that belongs to which ever form you are filing out. Ex: If the utilities expense for your workspace is $1000, and you use it 75% of the time for your employment, and 25% for your freelance work, then you would claim $750 on your T777 and $250 on the T2125.
FYI: To claim employment expenses on the T777, you would need a signed T2200 from your employer. If you have a T777S or no form at all, you would use the T777S to claim your employment expenses.