You may want to work through the design process in the FAQ.
http://wiki.arcadecontrols.com/index.php/FAQ#What_type_of_build_meets_my_needs.3FFocus on step 2 (game list) and see how those games display on the monitor you're considering using and what controls were on the original upright and cocktail cabs.
- You may not want to mix vertical and horizontal games.
- Some games alternate gameplay with P1 and P2 taking turns on a single set of controls and other games allow simultaneous gameplay with separate controls for both players.
There's a difference between setting the machine configuration/dipswitches to cocktail and the MAME "cocktail" video option.
- Machine configuration/dipswitches make the game inputs and responses behave like the original cocktail machines.
-- Some upright cabs alternated gameplay with P1 controls, but the cocktail cab had separate controls for both players and flipped the screen.
- The "cocktail" video option will usually
* give you two copies of the game display oriented toward the short sides of a WS monitor for horizontal games like Bubble Bobble but can be oriented toward the long sides for vertical games like Space Fury.
-- You may need to make custom layout files for 2 displays on vertical games
or rotate one big display and use cocktail flipping.
- Some games may not work well if you enable both machine configuration/dipswitches
and the "cocktail" video option.
* IIRC vector games will not work well with the "cocktail" video option.


Remember that there is no simple one-size-fits-all solution to make all MAME games look and work right on a cocktail build.
Scott