im not very experienced with streaming but i have done the setup so ill put a quick guide as i remember it. anyone feel free to add/correct...
join twitch
download/register xsplit
click broadcast on xsplit and edit the twitch.tv field with your twitch user/password
ARCADE: plug your camera in, on xsplit click add camera, you should see your cam, select it
MAME: open mame and start game (windowed mode) on xsplit click add screen region and click the mame window
you should now see your cam picture/mame window on the xsplit viewer, drag the corner to enlarge/fill the viewer
you can then click the view tab and set resolution as you see fit
click broadcast and select twitch and you should be live
during testing i had an issue with video/audio sync on aracde, you can click tools/setting there is an option to delay the audio part of the stream, it seems very awkward to get this correct yourself, it would help to have someone else listening to the stream and adjusting while you play the cab...