There is the SplitContainer.
The Splitter control was deprecated in winforms almost immediately. It was a single control that relied on docking to resize the left or top controls. It was a mess to manage when combining multiple splitters, or multiple controls.
The SplitContainer has two panels and can contain other splitters. The intersections should blend seamlessly (we have a bug regarding the intersection, wisej still shows both borders – but it’s an easy fix). Anyway, you’ll find the the SplitContainer is a lot better.
In addition, we have greatly expanded the basic SplitContainer and in Wisej each panel can have a title (see ShowHeader), the title can be oriented (see HeaderPosition), it can show a collapse button (see ShowCloseButton – like the GroupBox) and it can “spring” out when clicking or hovering on the title (see AutoShow property). It’s quite a complex container now.