I just recently had this same issue. I had to create our own ISection decorator that we would pass to the UltraGridDocumentExporter's export method. Add an IFlow on your new section, add 2 columns to it and keep a reference to the IFlow. When AddBand is called, you need to add the band to the IFlow and not the Section. After you export your first grid, call a method to add a flowcolumn break on your IFlow.Then export your second grid. You should change the pagesize yourself after the export is called, the exporter never seems to get that right anyway.

 public class ReportSection : ISection
    {
        private readonly bool _withFlowColumns;
        private readonly ISection _section;
        private IFlow _flow;

public ReportSection(Report report, bool withFlowColumns)
        {
            _withFlowColumns = withFlowColumns;
            _section = report.AddSection();
            if (_withFlowColumns)
            {
                _flow = _section.AddFlow();
                _flow.AddColumn();
                _flow.AddColumn();
            }
        }

public void AddFlowColumnBreak()
        {
            if(_flow != null)
                _flow.AddColumnBreak();
        }

public IBand AddBand()
        {
            if(_flow != null)
                return _flow.AddBand();

            return _section.AddBand();
        }

// All other implementations call the corresponding method on _section

}