Hello Dung,
the reason is that on Kinetis K there are two memory controllers, one for each memory area. Because it can cause a hard fault if you are accessing the memory boundary with an unaligned access, the two memory areas are splitted. If you ensure that you do not have any misaligned access, you can combine the two areas. See the comments section of FreeRTOS Heap with Segmented Kinetis K SRAM | MCU on Eclipse for a detailed discusson on that topic.
I hpe this helps,
Erich
