I need some help of creating shared ddr sdram memory

If you want the fastest memory transfer method, use an on-chip memory with a dual port and connect each port to a tight coupled memory port on each of the CPUs.

If you need more memory than what you have available in the FPGA, use external memory. If the external memory is an SSRAM, you can use uncached pointers or uncached macros. If the external memory is as SDRAM you will have a huge speed loss on consecutive accesses without a cache, so I would recommend to use the data cache and the flush functions. But in that case you must be sure that your memory blocks are aligned on cache lines, because as dsl says, nasty things happen when different blocks share lines and that you do flushes.