1) Is there a solution, both software and hardware, for 8051 to access 16-bit external data memory?
2) If it's possible, can you provide implemention in detail?
3) And, if to use 80251 instead of 8051 is more efficient in this case, or not?
1) WHY?
2) it is possible, the best solution depend on WHY
3) the 251 is still 8 bit externally, if you change chip use the Philips XA, it can be configured to 16 bit wide bus.

Erik