I've seen it reported differently elsewhere. I've seen where If SD and BOS split 50%, that each has about 7M against lux tax.
Though the extension Myers signed in Jan. 2017 calls for him to be paid $61 million ($20 million each season plus a $1 million buyout) through 2022, he’ll cost significantly less against the competitive balance tax threshold. Because Myers -- whose total deal was worth $83 million over six years -- has a backloaded deal, the CBT hit (which is based on average annual value) will be just $13.83 million in each season.
If the Sox took on half -- say, $30 million -- of Myers’ remaining salary, they’d take a CBT hit of less than $4 million in each of the next three years. The math is complicated, but that number comes from the money that would be sent from San Diego to Boston being equally spread over the remaining three years, lowering the CBT hit in each season. That scenario would be palatable for the Red Sox 2020, with their projected payroll then estimated to still be less than $200 million.
https://www.masslive.com/redsox/2020/02/red-sox-trade-rumors-boston-still-discussing-wil-myers-with-padres-also-interested-in-young-pitchers-report.html
Someone needs to teach a class on CBT math.