How to merge /data and /sdcard on Android 4.4? 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: /data and /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 /data with ext4 filesystem. Storage configuration in framework, fstab and 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.