以下でどうでしょうか?
void CMainFrame::OnSize(UINT nType, int cx, int cy)
{
CFrameWnd::OnSize(nType, cx, cy);
CRect rc;
mSplitterUp.GetClientRect( rc); *1
mSplitterUp.SetColumnInfo( 0, cx / 2, 0);
mSplitterUp.SetColumnInfo( 1, cx / 2, 0);
// RecalcLayout();
if (nType == SIZE_MAXIMIZED)
mSplitterUp.RecalcLayout();
}
void CMainFrame::OnSize(UINT nType, int cx, int cy)
{
CFrameWnd::OnSize(nType, cx, cy);
CRect rc;
mSplitterUp.GetClientRect( rc); *1
mSplitterUp.SetColumnInfo( 0, cx / 2, 0);
mSplitterUp.SetColumnInfo( 1, cx / 2, 0);
// RecalcLayout();
if (nType == SIZE_MAXIMIZED)
mSplitterUp.RecalcLayout();
}