I think the Simplified Method can still be used if you use a separate structure for your home office.
Having a separate structure for your home office WILL prorate your home sale, and the business portion would not be eligible to be excluded under the $250,000/$500,000 Principal Residence exclusion. Whether you use Simplified or Actual does not change that.
Yes, you can share a home office, as long as there is only used for qualifying purposes. If the office is shared 50%/50%, then EACH business can either (a) enter half the cost of all expenses (mortgage interest, utilities, etc.) and the full amount of square footage, or (b) the full cost of all expenses, and half of the square footage. But be aware of the 300 square foot limit if you use Simplified. I'm unsure how TurboTax deals with applying the mortgage interest and real estate taxes for your personal portion, so perhaps method (a) would prevent any potential problems with the program trying to double-apply things for your personal portion.
