No. Only one HD session and one SD downconversion.
You can always derive your SD NTSC and PAL masters from the finished HD master. A cost of downconversion to SD will be more affordable than a cost of another SD online edit session.
There may be rare cases where for technical or other reasons it pays to do a separate SD session. For example, if you’re able to output broadcast quality picture right from your SD offline NLE and you’re confident that your version is the same as the HD version you may save yourself a buck. This would apply only to the simplest of projects that do not use effects, color correction, titles, etc.