Problem resizing guest window on multiple monitor host

[iamthebull]

My host machine:
Windows 8.1 x64
VirtualBox 5.0.14r105127

My guest machine:
Windows 7 Pro SP1 x64
Guest Additions 5.0.14r105127

My host machine has 2 identical monitors configured for extended desktop. When I have Auto Resize Guest Display selected I am unable to resize the guest window larger than the width of a single monitor. For example, I would like the guest window to fill one monitor completely plus half of the other monitor. When I try to do this the guest window is automatically resized and fit to the width of one monitor. When I deselect Auto Resize Guest Display I can make the guest window any size I want.

Please advise.