How?
Assuming mmcblk1p2 is the ext4 (I don't see you mention this) then:

mount /dev/mmcblk1p2 /data

What?

Imagine device A has a filesystem with a dir (/data) and a file (/data/hello)

Essentially when you mount device B on a directory (/data) in device/filesystem A the OS looks into the filesystem on B for files instead of into the filesystem on A. So /data/hello can no longer be accessed at that path - it's hidden. Instead you'd see /data/file-hello-on-B

if you now mount device C onto the same directory it will look into the filesystem on C - now you'd see /data/file-hello-on-C

Usually this is a dumb thing to do since you may as well not bother mounting B.