>dt1
user     number
A        32
A        33
A        35
A        23
A        32
A        44
A        33

>dt2
user     number
B        32
B        33
B        35
B        23
B        32
B        44
B        33

>dt_combined
user     number
A        32
A        33
A        35
A        23
A        32
A        44
A        33
B        32
B        33
B        35
B        23
B        32
B        44
B        33
C        32
C        33
C        35
C        23
C        32
C        44
C        33