I'd use TJvSplitter from Jedi VCL, set their ResizeStyle = rsUpdate, and then on Splitter2's OnMoved event, I have this:
procedure TForm128.JvSplitter2Moved(Sender: TObject);
begin
Panel5.Width:=Panel3.Width
end;
and on Splitter3's OnMoved:
procedure TForm128.JvSplitter3Moved(Sender: TObject);
begin
Panel3.Width:=Panel5.Width
end;
This way, not only are they updated while dragging, but also follow along each other.