Sometimes I find the subscript is located too far from the letter (to which it is the subscript) such as in
How to get the subcript closer to the letter?
Reducing the size of the subscript is not the right solution, in my opinion. And
A "double subscript" is definitely not the best way to reduce the size, in any case.
If you really need to push a bit down the subscript, add an empty superscript.