The picture 16b shows the execution of instruction MOVX from external program memory. The instruction itself is FETCHED during PSEN low, and EXECUTION reads the byte from (normally another chip) external data memory while /RD is low. The same picture will be for WRITING to external data memory by MOVX, only /RD is of course high and /WR gets low. Unless you make HW tricks, this is a case of addressing two separate memories.

Regards, Slobodan