If you only have one USB available, then you should upgrade your computer.
My Laptop has Four USB Ports. You wouldn't buy a mixer with only one channel, would you...? My two lappies have 4 USB ports, Firewire, and eSata. If you are going to use a computer as a professional source for music playback -- don't get it at Wally World, and don't skimp on what you need...
I Custom/Special Ordered my Laptop from Dell complete with Top Of the line Sound Card at the time for a price of $1,500.00 in January 2006. What is so skimpy about that?
I'm not aware of any popular DJ software, that can handle only one sound card at a time...
I like the redundancy factor of using two cards. If one fails, you can switch both decks to the remaining working card on the fly (at least in my software you can).
I have several solutions for anything in my system that fails including soundcards. No need to restart the software, just a mouse click or two. I also like the fact of a DAC handling a single output at a time, and not multi-tasking.
Yer always hootin' and holerin' about using pro grade gear, yet it sounds like yer using a consumer grade computer.
See above statement. If that's the case, then you will have to make compromises...