I have the cheapest 4 port USB hub money can buy, and four of the cheapest USB gamepads I could find and have had no problem what-so-ever getting them all connected and working in mame.
This stuff is so cheap if doesn't even have a brand name - I guess even the back alley tiwanese factory / opium den that produced them didn't want to be assocated with such poor quality stuff. And yet there they are, working perfectly since the day I bought them.
So yes, it is very possible to have 2 USB devices in mame, I guess it depends largely on the manufacturer (or crack dealer) that made them.
Having said that...
My PC has 2 USB ports built into the motherboard - If I plug said cheap-arse controllers into the onboard ports I can only get one to work no matter how much I swear at it. For me an external USB hub makes all the difference.