So as noted the SSTB stuff doesn't matter unless taxpayer's taxable income exceeds $157,500 if single and $315,000 if married... and then even so, a phase-out range applies where someone slowly loses their Section 199A deduction as their taxable income moves from $157,500 to $207,500 if single and from $315,000 to $415,000 if married. The reality, then, is few professionals actually face losing Section 199A due to SSTB status.

But what you're talking about is the so-called Section 199A cliff. The final regs surprised some of us (or at least me) by making it VERY clear that having ten percent or more of a business do SSTB stuff taints the entire business as an SSTB. (That ten percent becomes five percent at $25 million in revenues.)

Here's cite for this: §1.199A-5(c)(1)

Also these three thoughts... First this cliff is really scary. We're finding in our practice that one needs to be really careful to look at whether or not someone's business includes ten percent consulting... or ten percent of anything that is SSTB activity. (We're trying to diligently document this for reason mentioned below...)

Second, a taxpayer may be able to say that he or she owns multiple businesses. But that should be obvious from facts. (Final regs talk about separate books and records. I think you want a clearly separate identity and operations, too, from reading final regs. Though specifically not required per final regs, separate entities would seem helpful.)

Third, while probably not workable for 2018, it seems really likely that some people falling over the cliff in 2018 might have their business restructured to clearly be two or more businesses in 2019 and going forward.

P.S. I did a way longer, practitioner-level discussion of the hassle of dealing with businesses that combine SSTB and non-SSTB activities that might be interesting for some: https://evergreensmallbusiness.com/section-199a-qualified-business-income-deduction-danger-zones/