I installed the last supported driver for GT 520:

391.35-desktop-win8-win7-64bit-international-whql.exe

As far as I can tell this did not solve any problems yet.

The GT 520 is not showing up.

At least now a driver is installed that should be able to support both drivers.
For now this is worthy of an experiment.

However it seems more like a hardware detection issue.

One thing I can think of is swapping the graphics cards.

Perhaps somehow the GT 1030 is detected first and some how disables the GT 520.

But first I will try a re-boot and see if there is anything in the bios that might be causing the GT 520 to not be detected.

Bye for now,
Skybuck.