There is not a place to enter a percentage to split the home office use between two businesses.  You will need to divide the square feet of the office space between the two businesses instead. 

See the information below for some additional guidance about claiming a home office space for more than one business.  This is from the TurboTax help content.

Used this office space for more than 1 business?

So you have more than 1 business, and you use your office not just for this business, but for at least one other business.

First of all, on the "Enter the Square Footage of Your Home and Home Office" screen you saw earlier, make sure you divided up the square footage correctly so that each business gets the correct amount of square footage for the year.

Now, for your expenses:

Enter the full amounts you paid during the time you used the space for either office. Do the same on the other office.

It might seem like you're entering everything twice, but since you divided up the square footage earlier based on how you use it, all of the expenses for the total square footage will be accounted for on your tax return.

We'll calculate the correct expense amounts for each office, and the total expense amount will then be correct on your tax return.