If I already use 20% corporate business use portion for my home, is it wise to deduct additional for my sole proprietor business?

Although we cannot provide you with "tax planning advice" as to whether or not you should be making this claim, we can advise you of the rules involved with making such a claim.

If you have separate business activities and they both meet the qualifications for a home-office deduction, you must file a separate tax form for each business. (In your case your corporate business is a separate entity anyways on the T2 form). 

• If you use a separate office space for each activity, simply figure out the percentage of your home that each space takes up, and use those figures when calculating your deductions on the forms. 
• If you use the same space for both activities, you can split the deduction between the two activities.