I know the quesiton was not new, but I just ran into the same issue.and found with search. Here's an alternative method to get around the problem.
In turbotax manually enter the 1099 DIV in two parts. One contains only the CG entry (for tax return this means only LT CG). The other contains only the Dividend entries (which for tax return includes Div and ST CG). Make the correct adjustment in each part. I append the 1099 "reveived from" name with CG and DIV to avoid duplicate names.
I have been using this approach for years for similar circumstances. e.g. separating adjustments to tax exempt and taxable interest.
Step by step can be opaque to on details so I check result using forms view.
