Yes, I suppose so. There is clear authority to adopt a plan retroactively without even invoking -11(g). There is no question adopting a new plan is permissible, the question is whether B adopting A's plan comes under this new authority.
Ideally if -11(g) was being drafted or amended post-SECURE, it would address this. Section 201 of SECURE says "the employer may elect to treat the plan as having been adopted as of the last day of the taxable year." Who is the "employer" in this regard, each separate entity or the aggregated CG employer? Either way you consider "employer", adopting a new plan retroactively works. OTOH if you consider the "employer" to be the aggregated entity, it already sponsors the existing plan, which could imply that adding B is more of an amendment than an adoption. Of course if it is an amendment then why can it be done under authority of -11(g) (except maybe the deductibility is not retroactive?). But then if each entity is filing separate tax returns then maybe B adopting A's plan is an amendment for -11(g) purposes, but then B as a separate taxpayer can invoke 401(b) as amended and elects to treat its adoption as being done as of the last day of the year for deductibility.
IMO it should work per my second paragraph, but it might need to be defended under exam, and if an employer has to fight the IRS on this they've already "lost" in terms of dollar cost, anxiety and distraction. So I'd advise a client in this situation to adopt a separate plan now to avoid the issue. But if I came across a client that did it the other way I wouldn't tell them it's wrong, I'd just explain there's no guidance, if you put the pieces together it should be OK, but the pieces came out at different times and IRS may not agree.