Multiple Web Viewers And AsyncRendering True

I have multiple viewers on a web page and initialy would like AsyncRendering = true on all viewers, so I can have the loading message. Once the user drill-through, I hide all other viewers and change to AsyncRendering = false, so the autosizing will work.
But, with AsyncRendering = true (in code behind):
private void SetDominateViewerProperties(ReportViewer rptViewer)
{
rptViewer.AsyncRendering = false;
rptViewer.ShowToolBar = true;
rptViewer.ZoomPercent = 100;
rptViewer.Visible = true;
}
private void SetDefaultViewerProperties(ReportViewer rptViewer)
{
rptViewer.AsyncRendering = true;
rptViewer.ShowToolBar = true;
rptViewer.ZoomPercent = 100;
rptViewer.Visible = true;
}
I have only one viewer showing, even though, I can see in debug that all viewers are set to visible. In SetDefaultViewerProperties, I change rendering to false, I get multiple viewers. Are there combinations of property values I need to set?