split wall
Assuming you want to split the Wall vertically, you can use the BREAK command on the Wall. If you select the Wall at the point where you want to split it, you can type @ when prompted for the second point to split the Wall with no gap. Otherwise, if the second point is different from the first, there will be a gap between the two resulting Walls.
If you hover over the cursor over the Solution Tip Icon, you should get a tool tip indicating what the problem is and offering potential solutions. This Autodesk Knowledge Network Article has the Help information on common Solution Tip situations. The image below shows the tool tip in action (although this is unlikely to be the issue you face after splitting a Wall). If you can WBLOCK the Walls involved out to a separate file and if the Walls continue to have the cleanup issue in that file, you can post that as an attachment to a reply here and someone here can take a look at it and make suggestions for fixing the condition.
Walls can be stacked.
How do you want them to look when you split horizontally?
Could you provide an elevation view of what you want in the end.
If that green line is horizontal, you can create a single Wall Style that has one set of components up to the green line and another above it. Components do not have to run from Wall Bottom to Wall Top.
It depends on the purpose of the split in construction terms.
1. If you want to change material "color/hatch" or wall thickness, then @David_W_Koch mentioned you can stack components in walls.
2. A reveal line can also be added using a sweep if you just want a visual break for your elevations.
3. walls can also be stacked.
