I'm not a berryboot user, but if you have two root fs partitions on the card, probably only one of them will be mounted when that OS boots.
This may be a bit confusing because in a GUI environment, adding (=mounting) devices (such as a USB drive) is done automatically when those are plugged in, probably including when they are already plugged in at boot time. However, unless explicitly configured otherwise, partitions on the SD card will be ignored. For example, usually on the Pi the boot partition is configured to be mounted, even though it is not used except for updating the contents (kernel, firmware, etc).
To see what partitions are available on the card:
ls /dev | grep mmcblk0
You have (at least) three;
mmcblk0p1 is the boot partition, the other two are root fs partitions (berryboot may also require a small one for its purposes). To see which are mounted:
mount | grep mmcblk0
To mount one that isn't:
sudo mkdir /mnt/whatever
sudo mount /dev/mmcblk0p2 /mnt/whatever
/mnt/whatever directory (you don't have to use that path) should now contain the contents of the partition.
To make that happen automatically at boot, add a line to
/dev/mmcblk0p2 /mnt/whatever ext4 defaults,noatime 0 3
The filesystem type (
ext4) is a presumption; there should be oodles more stuff about fstab online.