Grumpy_Mike:
Sorry but no. The one file only is a restriction of the SD card technology, nothing else.
Oh Mike, you should know better than this.
With a more powerful board and more sophisticated library design, of course 2 or more files can be played simultaneously from a SD card. My audio library for Teensy does it easily, with two stereo files at 44.1 kHz sample rate. Here's video evidence, if you or anyone else is skeptical (start viewing at 9:06, if this link doesn't automatically begin 9 minutes into the video).
The restriction isn't SD card technology at all. It's the limited performance of 8 bit AVR and its lack of sophisticated peripherals & DMA, with well designed libraries to efficiently stream with those peripherals.
When your processor is just barely fast enough to read one audio file and it consumes nearly 100% of its CPU time (and most of its RAM) doing so, without any way to efficiently process and output that data other than tightly synchronized to output the samples as they're read, then of course you can't play more files simultaneously. But that's not the SD card technology holding you back. It might seem that way if you only ever work with such very limited microcontrollers and fixed-function modules which support 1 file at a time (or the simplistic example code Arduino publishes for Due & Zero), but in fact that very capable SD card is just sitting there responding to the very slow read speed and limited software of regular AVR-based Arduino.
So much more is actually possible with SD cards, and audio in general, when using more capable hardware.