Its actually one main driver. The other one you see is a filter driver -- iaStorF.sys, where F means filter.
A filter driver is a Microsoft Windows driver that extends or modifies the function of peripheral devices or supports a specialized device in the personal computer. It is a driver or program or module that is inserted into the existing driver stack to perform some specific function.(wikipedia)
Here is a screenshot of my system, for disk drives, its in
The main driver is disk.sys, the other 3 are all filter drivers, EhStoreClass.sys and partmgr.sys are default MS drivers in Windows 8, edevmon.sys is from eset.
For monitors, the additional inf is optional, used for extending the capacity, like color profile(.icm) or EDID information, these inf are not required for monitor to operate, only monitor.sys is necessary. (MSDN)
Some more info about filter driver, I'll just reproduce it here for future reference.(MSDN)
Filter drivers are optional drivers that add value to or modify the behavior of a device. A filter driver can service one or more devices.
Bus Filter Drivers
Bus filter drivers typically add value to a bus and are supplied by
Microsoft or a system OEM (see the Possible Driver Layers figure). Bus
filter drivers are optional. There can be any number of bus filter
drivers for a bus.
A bus filter driver could, for example, implement proprietary
enhancements to standard bus hardware.
For devices described by an ACPI BIOS, the power manager inserts a
Microsoft-supplied ACPI filter (bus filter driver) above the bus
driver for each such device. The ACPI filter carries out device power
policy and powers on and off devices. The ACPI filter is transparent
to other drivers and is not present on non-ACPI machines. Lower-Level
Lower-level filter drivers typically modify the behavior of device
hardware (see the Possible Driver Layers figure). They are typically
supplied by IHVs and are optional. There can be any number of
lower-level filter drivers for a device.
A lower-level device filter driver monitors and/or modifies I/O
requests to a particular device. Typically, such filters redefine
hardware behavior to match expected specifications.
A lower-level class filter driver monitors and/or modifies I/O
requests for a class of devices. For example, a lower-level class
filter driver for mouse devices could provide acceleration, performing
a nonlinear conversion of mouse movement data. Upper-Level Filter
Upper-level filter drivers typically provide added-value features for
a device (see the Possible Driver Layers figure). Such drivers are
usually provided by IHVs and are optional. There can be any number of
upper-level filter drivers for a device.
An upper-level device filter driver adds value for a particular
device. For example, an upper-level device filter driver for a
keyboard could enforce additional security checks.
An upper-level class filter driver adds value for all devices of a