I have two top guns on my mame cabinet as well. It wasn't terribly hard to setup. Do you have the sensor bars installed already and the drivers working? Offscreen reload is really the only problem I have in some games, Lethal Enforcers especially. If I remember correctly you have to setup a key in the hkems control panel for reload. You have to calibrate the guns outside of mame and you may have to calibrate again inside of each game. Probably get more info over at forum.arcadecontrols.com. Edit your mame.ini as follows
#
# CORE INPUT OPTIONS
#
lightgun 1
offscreen_reload 1
#
# CORE INPUT AUTOMATIC ENABLE OPTIONS
#
lightgun_device mouse
#
# INPUT DEVICE OPTIONS
#
dual_lightgun 0