Windowed mode won't use dual monitors?

[ChamATX]

I've got a Windows 7 Pro machine with two 1600x1200 monitors. I have VirtualBox 4.3.16 installed, with Oracle Linux 6.4 running as a guest OS.

My problem is that I cannot get the guest OS to really span across monitors in "Windowed" mode, with auto-resizing turned on. I can resize the guest OS all I want, as long as the window completely fits on one monitor. As soon as I try to resize the guest VM window across to fit across both monitors, the guest VM window "snaps" back to fit in one monitor. It's very frustrating!

Note that in "Seamless" mode, I can make use of both monitors. But no application window within the guest can actually sit on two monitors at the same time. If I drag a window by its title bar onto both monitors, the only portion of the window that is visible is the part that sits on the same monitor as my mouse cursor. The part on the other monitor is not visible, until the mouse cursor itself crosses onto the other monitor, at which point, the part of the window on that monitor suddenly appears, and the part that was on the monitor I my cursor just left disappears entirely. When I let go of the window, it remains as it was-- part visible, and part not visible. I can move my mouse cursor anywhere at this point, and the window is unaffected.

Scaled mode is the only thing that "kind of works", as I get two separate windows for the VM, and I can position them side by side on the two monitors. But that's not really ideal, either-- I lose some real estate for the window borders of both windows, and it's virtually impossible to get the scaled display to exactly match the monitor resolution so that reading text is easy. (Scaled text is always blurry and hard to read...) That's just not usable for much of anything other than testing a Guest OS's support for multiple monitors...

Any ideas or suggestions?

[akaAlso]

I've been having problems with recent releases and multiple monitors.
I use windowed mode and extend by dragging the main window on my 2560x1600 across onto one of the 1200x1600 either side.
The issue was usually a BSOD crash if the system went into standby or sometimes when I tried to resize the window once I had programs running in the VM.
Recently tried the update going from 4.3.12 to 4.3.20 which worked briefly but then refused to allow me to manually stretch across monitors and resulted in more crashes.
I've now reinstalled 4.3.2 - not sure this is enough of a rollback but so far so good.

ETA - Seems stable so I'll be sticking with this version. I have raised a bug ticket for my issue.