This is, SQL-wise, not really a join, but a union. Which makes for rather complicated SQL, as you have to repeat the subquery for data from HAVE1.
I think it is a lot easier with a data step, e.g.:
data want;
found=0;
do until(last.IDnumber);
set have1(in=in1) have2(in=in2);
by IDnumber date;
if in1 and found=0 then do;
found=1;
output;
end;
else if in2 and found=1 then
output;
end;
drop found;
run;
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