How to generate multiple files from an IDOC with multimapping in SAP PI 7.3.1

We have an IDOC to file scenario where multimapping is implemented to create 3 different files from a single IDOC, with their destination folder on 3rd party system being the same.

We now see another requirement to generate these 3 files (say x,y,z) in pairs with similar filenames, one being x.txt with its pair as x.trg. Similarly, we would need to generate y.txt and y.trg, z.txt and z.trg. Please suggest on how to implement the same. We tried Variable Substitution of no luck - We're unsure if this works for multimapping.

Please suggest.

Thanks in Advance.,

Manasa