... would I have no choice but to file "married filing separate" in order to do that?
Not sure how YOU would file. Tax returns are like snowflakes, no two alike.
My wife and I, both retired from previous careers, each have a single entity LLC. I have one, she has one. I am an options trader with TTS, she owns a small one person travel agency in which she arranges travel and tours for folks who want to travel to Ireland (she's from Galway.) Our accountant, who is both a CPA and Tax Attorney says that according to IRS;
For Multiple Businesses
If you have separate business activities and they both meet the qualifications for a home-office deduction, you must file a separate Form 8829 for each activity. 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.
For Multiple Users
If you and another person both use the same home office space for qualified business activities, you should split the deduction between the two of you. When splitting the deduction, you should consider factors such as how much time each person uses the space and how much of the space is dedicated to each person's equipment and storage.
So when our CPA does our taxes he prepares;
*two Schedule Cs one for each of our businesses [Schedule C, Profit or Loss From Business (Sole Proprietorship)],
*two Form 8829s [Expenses for Business Use of Your Home] (we split the expenses),
*one Form 6781 [Gains and Losses From Section 1256 Contracts and Straddles] and;
*two Form 1099-Rs [Distributions From Pensions, Annuities, Retirement or Profit-Sharing Plans, IRAs, etc.]
*one Form 1040 [U.S. Individual Income Tax Return]. We file jointly and for Line 12 he files both Schedule Cs.
So far that has worked. We pay the taxes due, the CPA gets a fee, we're happy, he's happy, we've not been challended since 2013, and the IRS is....(add your own description of the IRS ______________________________________.)
Best
For more information see:
Tax Topic 429 - Traders in Securities (Information for Form 1040 Filers)
https://www.irs.gov/taxtopics/tc429.html
https://www.irs.gov/businesses/smal...yed/single-member-limited-liability-companies
https://www.irs.gov/help-resources/tools-faqs/faqs-for-individuals/frequently-
asked-tax-questions-answers/small-business-self-employed-other-business/entities/entities
Expenses for Business Use of Your Home
https://www.irs.gov/pub/irs-pdf/f8829.pdf
Profit or Loss From Business (Sole Proprietorship)
https://www.irs.gov/pub/irs-pdf/f1040sc.pdf
Distributions From Pensions, Annuities, Retirement or Profit-Sharing Plans, IRAs, etc.
https://www.irs.gov/pub/irs-pdf/f1099r.pdf
Also see;
6 U.S.C. United States Code Title 26 - INTERNAL REVENUE CODE
Subtitle A - Income Taxes
CHAPTER 1 - NORMAL TAXES AND SURTAXES
Subchapter P - Capital Gains and Losses
PART IV - SPECIAL RULES FOR DETERMINING CAPITAL GAINS AND LOSSES
Sec. 1256 - Section 1256 contracts marked to market
From the U.S. Government Printing Office,
www.gpo.gov
https://www.gpo.gov/fdsys/granule/U...tle26-subtitleA-chap1-subchapP-partIV-sec1256