The 8051 is ideally suited to the type of manipulation you desire. There are most likely dozens of ways to code it. Here is one idea.

     MOV    A, #00FH    ; low nibble mask
     ANL    A, R1       ; A has low nibble of R1
     MOV    R0, A       ; save for a moment in R0
     MOV    A, #0F0H    ; high nibble mask
     ANL    A, R2       ; A has high nibble of R2
     ORL    A, R0       ; combine high with low nibbles

Michael Karas