I think either is acceptable, though with one man plan, I believe it is more common to open a separate ROTH brokerage sub-account to easily track the G/L between sources and transfer from the non-roth to the roth on the conversions.
and if the brokerage house will be responsible for 1099-Rs I would absolutely want to set up a 2nd account for the ROTH piece rather than try to argue with them when the eventual distributions are done.