How to control Windows 7 snap feature with two monitors?

You should use only keyboard shortcuts for that configuration, since mouse movements alone seems not supported for that special side that crosses over to the other monitor.

NOTE: On multiple monitors, continually pressing a keyboard shortcut combination below will cause the window to snap to the side and scroll in that direction through the multiple monitor screens.

• Windows Key + ←
  This will snap the current window to the left side of the screen, or unsnap a window that is snapped to the right side of the screen.

• Windows Key + →
  This will snap the current window to the right side of the screen, or unsnap a window that is snapped to the left side of the screen.

For what it's worth there is a useful tool which runs on Windows called WinSplitRevolution (*) which offers further window management features, including the ability to 'throw' windows to the left/right monitor or to resize windows to the left hand side, centre, top, right of the screen and etc. I find the Windows 7 default keys useful, this tool is much more comprehensive though.

(*) Later edit, unfortunately this tool is no longer being actively maintained, see the website for details.

MaxTo now contains a snap-to-window feature for Windows 7, as well as being a superb general window manager.

Version 2010.12

Released December 17th 2010.

New features

Windows snap to other windows, monitor edges and regions. Disabled by default.

As of 2021 January, MaxTo costs €29 for a lifetime personal license and €19/user/year for a business license.