Need help configuring dual monitors

[clarkleach]

*** UPDATE ***
I installed a new kernel from backports (4.9.0). This certainly made a difference. But there are still issues...

1. The option to select kernel 4.9.0 does not appear in the grub menu unless I enter the BIOS boot menu first and select 'debian' as my boot device. I can then select which kernel I want to boot. Does this mean I need to run grub-mkconfig?

2. I now have a desktop extended to both monitors. But not without issues. I've noticed tearing when resizing (normalizing a maximized) window. So far I've only noticed this in Chrome. Is this maybe a 'feature' of the GLX renderer or Display Server (neither one of which was upgraded with the new kernel)? The only change in the output of 'inxi -F' seems to be the 'drivers' item which I'm not sure how to interpret:

with 3.16.0-4-amd64 kernel:

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Graphics:  Card: Intel Device 191d
           Display Server: X.Org 1.16.4 drivers: fbdev,intel (unloaded: vesa)
           Resolution: 2560x1440@93.00hz
           GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.5, 256 bits) GLX Version: 3.0 Mesa 10.3.2

with 4.9.0-0.bpo.2-amd64 kernel:

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Graphics:  Card: Intel Device 191d
           Display Server: X.Org 1.16.4 drivers: intel (unloaded: fbdev,vesa)
           Resolution: 2560x1440@59.95hz, 2560x1440@59.95hz
           GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.5, 256 bits) GLX Version: 3.0 Mesa 10.3.2

Also there is a very small but noticeable overlap where the two screens meet, meaning the same thing appears on both monitors - a 'feature' I had noticed before on my laptop running Deb 8.2/Mate. Maybe Display Server or GLX Renderer related? Is there a prudent way to force an upgrade of the Xorg display server and change the GLX renderer to match that of Kubuntu to see if these problems goes away?

Kubuntu (4.8.0-51-generic):

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Graphics:  Card: Intel Device 191d
           Display Server: X.Org 1.18.4 drivers: (unloaded: fbdev,vesa)
           Resolution: 2560x1440@59.95hz, 2560x1440@59.95hz
           GLX Renderer: Mesa DRI Intel HD Graphics P530 (Skylake GT2) GLX Version: 3.0 Mesa 12.0.6

blah, blah, blah....