Thinking about this some more, the behaviors are likely different between the two cases.

For two snapped windows, there's utility in the current implementation. Not only does it swap sides but it allows you 'unsnap' that first snapped window, allowing you to swap it out in a way that you can't currently do without closing splitscreen.

Because overview would be open in the single snapped case, that would mean swapping the side overview was on, which means changing the side of the first snapped window, but keeping it as the 'locked' one.

This means we either have these two inconsistent behaviors depending on whether or not overview is open, or we lose the lock swapping utility of the current implementation. I think the former is preferable, but not 100% sure. Omri, WDYT?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7895b694c73f692c46052faad1f5197090fd52e9

commit 7895b694c73f692c46052faad1f5197090fd52e9
Author: Xiaoqian Dai <xdai@chromium.org>
Date: Tue Apr 10 00:45:20 2018

Cros splitscreen: Double tapping behavior.

Currently double tapping only works when there are two snapped windows,
and double tapping will swap these two windows' positions. This CL makes
the double tapping work for single snapped window case, i.e., the other
side of the screen is occupied by overview. Double tapping will swap the
position of the snapped window and the overview window grid.

Bug:  827422 
Change-Id: I5c7208d6e10d76066ae531b9f9652631f4ec6818
Reviewed-on: https://chromium-review.googlesource.com/1003047
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549352}
[modify] https://crrev.com/7895b694c73f692c46052faad1f5197090fd52e9/ash/wm/overview/window_selector_unittest.cc
[modify] https://crrev.com/7895b694c73f692c46052faad1f5197090fd52e9/ash/wm/splitview/split_view_controller.cc