I think dual screen emulation in games have been officially supported since mame versions dating back to around June of last year, I don't know the list of games that are dual screen though, hehe I should really look into it since I made such a cab, but I have Punch-Out & Super Punchout and the Playchoice 10 games working dual screen so far.
I think most decent dual head graphics card should output to 2 lcd's or crt monitors just fine, my older ATI Radeon 9600xt does this as does other Radeons and some Nvidia and I've also heard good things about Matrox cards too. I think the description of the graphics card will say dual monitor support, or dual-link dvi support, and have two dvi's or a dvi + vga on the back.
However, in my case I needed two graphics cards since I'm using a TV as primary and lcd monitor as secondary, and the quirky TV-Out feature of my 9600 sends the same signal to the lcd (set as a second monitor) as it does the primary tv and it was a weird frequency a monitor couldnt seem to handle.
Then after hooking up both screens and setting one to primary, one to secondary it was just a matter of making an INI folder in the main MAME directory and making a copies of my mame.ini ( learned this from HowardC) renamed to punchout.ini or whichever dual screen game and the numscreens value set to 2, so only those games utilize the 2nd monitor and everything else plays on the primary as normal.