data m;
         
             set m1  ;
                     do i=1 to nobs;
              set m2 nobs=nobs point=i;
           if newv5 = v5 and newv6 = v6 then output;
          end;
      
run;

The set, merge etc. statement can open many data sets.

How do you know it 只读一次？

The following log says two observation have beed read in.

161  data m1 m2;
162  x=1;output;
163  x=1;output;
164  run;

NOTE: The data set WORK.M1 has 2 observations and 1 variables.
NOTE: The data set WORK.M2 has 2 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.01 seconds
      cpu time            0.00 seconds


165
166  data m;
167               set m1  ;
168                       do until (last);
169                set m2 end = last;
170             *if newv5 = v5 and newv6 = v6 then output;
171            end;
172
173  run;

NOTE: There were 2 observations read from the data set WORK.M1.
NOTE: There were 2 observations read from the data set WORK.M2.
NOTE: The data set WORK.M has 1 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.01 seconds
      cpu time            0.01 seconds