I did a search trying to find out how one can typeset multiple lines under a sum and found out that there is the possibility to use
\substack together with a remark that one shouldn't use
Is there any good reason for that? Does it commit some typographical sins I am not aware of?
\atopis abusing its functionality; if you want to turn a screw, sometimes a knife can help, but a screwdriver is surely better.
In other words, use
\substackthat has been specifically defined for this task and can accommodate any number of lines. It's also easier to use even for two lines.
The first is wrong; the second is complicated to write. With
\substackyou also avoid the warning