@dmertz - I was concerned about that, so I had already tried the split 1099 approach and it it resulted in the exact same result on Form 8606 Part II as my approach (my situation does not involve Form 8606 Part III). I believe that no impact on Form 8606 is correct because Form 8606 Part II is only used for conversions from IRAs (conversions from 401(k)s are not reported in Part II) and Part III line 24 includes rollovers from 401(k)s in addition to amounts previously reported as basis on Form 8606.
