The only experience I have with dual monitor systems are Mac sound editing
systems, so this is pretty much speculation, but:

It seems to me that two "real" uses for multihead systems are 1) Expanded
desktop real-estate, as with the sound editing, and 2) running a process in
one window while controlling or monitoring that process in another, when you
need full-screen or near-full-screen window size in one or more of them.

When it comes to emulators and running multiple processes, I have yet to see
a PC grade system where it wasn't a better idea to set up two different
boxes and run the different processes on seperate hardware.

It's true that I've never had the option to spend the cost equivalent of a
second system on pumping up a single system - I've never gone to gigabyte
RAM or multi-processors.  I know enough about system tweaking though to
predict that for a given budget, you can get more performance on two
processing tasks from two different systems than from one system on
steroids.

I'm not talking about the kind of basic multitasking that we do most of the
time - multiple windows running different programs, opening multiple files.
I did that with DOS and DesQview before Windows95 made them obsolte.  Most
of the time, for most of us, we may have one system intensive process - a
compile, a sort, a download - and are running little "chores" in other
windows.  When we try to run more than one primary process on a PC it's
usually going to take about as long to run them in parallel as it would to
run them in sequence.  If we have a need to do this on a regular basis, it's
time to build two boxes.  Limitations of system IO,  bus processing, etc.
just don't allow PC grade hardware to do anything else.

I do plan for  my next system to be a Linux primary system, probably with
Crossover Office for MS programs I still need.  I won't, however, be trying
to emulate a PC on a Linux box (or vice-versa).  That kind of work is
inherently frustrating and the only people who seem serious about it are
software developers who need or want multiple environments, and don't
actually need production level processing on the emulated system.  As great
as VMWare might be, it's still an emulation, and that means that you have a
primary OS, an emulation environment, and the emulated system as overhead,
and you just can't get around that.

The hardware we (most of us) have access to these days is optimized for
single-interface, single-user work.  It's utterly capable of supporting a
number of text-based consoles just as well as the mid-sized computers of
days gone bye, but with the exception of top of the line systems most start
to drag their feet when a GUI is loaded, and the thought that performance
would be anyting but a novelty with multiple GUI users on a PC is ludicrous.

The only successful multi-user GUI systems I know of use "terminals" that
cost several times what a PC costs and have more processing power at the
workstation than most PC's.