Snapping windows in  #openbox  
#obkey  makes configuring keyboard shortcuts much easier.

Click 'Insert sibling keybind'
Give it a combo (Super / Meta / Windows + left arrow key)
Insert actions:
- UnmaximizeFull
- MaximizeVert
- MoveResizeTo (Set width to half your screen resolution and x coordinate to 0)

Repeat for (Super + right arrow key) except set the x coordinate to half your screen resolution.