Is it possible to use two partitions as one storage on a MediaTek device?
AFAIK, it is already implemented in Samsung Galaxy S III, may be.
Not very large number of physical primary only devices were shipped with separate
/sdcard partition (with attribute
removable="false") in transition phase from UMS to MTP. Most of those were shipped with ICS or JB, a few got updated to KK. Then mostly vendors moved to the dominant concept of emulation on
/data/media/0. See Android's Storage Journey for more details.
You can take risk to paly with partition table in order to merge both partitions:
/sdcard (depends on the layout of partitions). But how would you get a compatible ROM? In fact you'll have to develop a custom ROM from scratch, which needs modification to both; AOSP and kernel code. Particularly
vold configurations need special upgradation as it would be looking for partition to be mounted at
/sdcard with (permission-less) FAT filesystem, which is now merged to
ext4 filesystem. Storage configuration in framework,
init.*.rc files would also needed to be changed.
So the struggle required isn't worth it. And I doubt even if you would be able to get complete source code for such an old device.