If you're using .NET 2.0, the SplitContainer would be the right tool for the job.Put it on the form, set its Dock property to Fillan Orientation property to Horizontal. Put the first grid in the upper panel and secondgrid in lower one. Set both grid's Dock panel to Fill.
If you're using .NET 1.x, the Splitter control will help you achieve a similar solution. Just dock it between both grids.
Andrej