I'm just getting into this myself but from what I understand there are three types of tables now... desktop (the old way), fullscreen (no backglass, for vertical monitors), and fullscreen spanned (backglass on the left, table on the right)
If shown in "windowed" mode you should be able to use the fullscreen spanned on a dual monitor setup by putting your backglass monitor to the left... virtually speaking of course.
Also keep in mind that there are these "active backglass" packs that you can use with regular fullscreen tables. Yes you have to download them seperately... check vpforums.org
Also to confuse things even further, there are 3 or 4 versions of pinmame now.... the regular one we all know and hate and special "hacked" versions that are more friendly towards the three different setups above.
Again, vpforums.org