How to make a simulated SD card (an image) work together with a real one in Android 2.2?

I've managed to simulate a fake sdcard on Android 2.2, so that when I don't have a real sdcard, applications that need sdcard will still work. To achieve this, I borrowed some code from the Andr开发者_开发百科oid-x86 project's extended version of `vold', and did the following:

# dd if=/dev/zero of=/data/sdcard.img bs=1024 count=1 seek=1000000 (1GB image, a sparse file)
# busybox losetup /dev/block/loop7 /data/sdcard.img
# newfs_msdos /dev/block/loop7

The problem is, when a real sdcard is inserted, I wish to use both of them without switching back and forth between them - namely, a `dual sdcard setup' on a system which only allows one sdcard. How can this be achieved?

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Personally I'm not quite familiar with the Android system (actually today is my second day working on an Android project). I was told that the 2.2 version only supports one sdcard mounted on "/mnt/sdcard", applications that read/write sdcard will follow this path. I'm not quite sure about this. Can someone please shed me some light on the Android sdcard limitations and how its presence affects applications?