BCGControlBar Pro for MFC
void CBCGPPropList::EnableProportionalColumnResizing ( BOOL  bEnable = TRUE)

Specifies whether the control's columns are resized proportionally to the recent widths when the control is being horizontally resized.

Parameters
bEnableSet this parameter to TRUE to enable the proportional column resizing. If this parameter is FALSE, the columns will always have the same size (50/50) after the resizing.