Your solution is easy enough, you just have to follow the jsl syntax. Save() requires a string argument so you must be careful with where you place the quotation marks (reading the error messages in the log is good for learning how to debug your code).
And you must concatenate the substrings of course.
e.g.
id1||"_"||id2||".jmp"
If you have numeric columns you must also use Char()
char(id1)||"_"||char(id2)||".jmp"